tec: Update the lib_opml (#4403)

* fix: Fix undefined GLOB_BRACE on Alpine

The manual states that:

> Note: The GLOB_BRACE flag is not available on some non GNU systems,
> like Solaris or Alpine Linux.

This generated an error on Alpine.

Reference: https://www.php.net/manual/function.glob.php

* fix: List details of feeds for OPML exportation

The details are necessary to export the XPath information, the CSS full
content path and read actions filters.

* Update LibOpml to 0.4.0

* Refactor OPML importation to be more robust

First, it fixes two regressions introduced by the update of lib_opml:

- title attribute is used when text attribute is missing;
- the OPML category attribute is used as a fallback for feeds categories.

In a related way, if also fixes a problem when a feed had both a parent
category outline and a category attribute. Before, it only considered the
attribute as its category, but now it considers the parent outline.

Then, it counts category limit correctly by not increasing
`$nb_categories` if the category already exists.

* Exclude lib_opml from the CodeSniffer

* Fix variable names when logging some errors

* Fix catch of LibOpml Exception

* Make sure to declare the category

* Exclude lib_opml from PHPStan analyze

* Disable markdownlint for lib_opml

* Fix typos

* Use auto-loading and allow updates via Composer

* Fix broken links to lib_opml

* Bring back the ability to import the OPML frss:opmlUrl attribute

* Refactor the logs of OPML errors

* Update lib_opml to the version 0.5.0

Co-authored-by: Alexandre Alapetite <alexandre@alapetite.fr>

12 files changed, 1266 insertions, 353 deletions

diff --git a/lib/.gitignore b/lib/.gitignore
index 812bbfe76..a1df80381 100644
--- a/lib/.gitignore
+++ b/lib/.gitignore
@@ -1,6 +1,14 @@
 autoload.php
 composer.lock
 composer/
+marienfressinaud/lib_opml/.git/
+marienfressinaud/lib_opml/.gitlab-ci.yml
+marienfressinaud/lib_opml/.gitlab/
+marienfressinaud/lib_opml/ci/
+marienfressinaud/lib_opml/examples/
+marienfressinaud/lib_opml/Makefile
+marienfressinaud/lib_opml/src/functions.php
+marienfressinaud/lib_opml/tests/
 phpgt/cssxpath/.*
 phpgt/cssxpath/composer.json
 phpgt/cssxpath/CONTRIBUTING.md
diff --git a/lib/composer.json b/lib/composer.json
index 4e4e1c051..6e9e0ee32 100644
--- a/lib/composer.json
+++ b/lib/composer.json
@@ -12,6 +12,7 @@
     ],
     "require": {
         "php": ">=7.2.0",
+        "marienfressinaud/lib_opml": "0.5.0",
         "phpgt/cssxpath": "dev-master#4fbe420aba3d9e729940107ded4236a835a1a132",
         "phpmailer/phpmailer": "6.6.0"
     },
diff --git a/lib/lib_opml.php b/lib/lib_opml.php
deleted file mode 100644
index f86d780b7..000000000
--- a/lib/lib_opml.php
+++ /dev/null
@@ -1,353 +0,0 @@
-<?php
-
-/**
- * lib_opml is a free library to manage OPML format in PHP.
- *
- * By default, it takes in consideration version 2.0 but can be compatible with
- * OPML 1.0. More information on http://dev.opml.org.
- * Difference is "text" attribute is optional in version 1.0. It is highly
- * recommended to use this attribute.
- *
- * lib_opml requires SimpleXML (php.net/simplexml) and DOMDocument (php.net/domdocument)
- *
- * @author   Marien Fressinaud <dev@marienfressinaud.fr>
- * @link     https://github.com/marienfressinaud/lib_opml
- * @version  0.2-FreshRSS~1.20.0
- * @license  public domain
- *
- * Usages:
- * > include('lib_opml.php');
- * > $filename = 'my_opml_file.xml';
- * > $opml_array = libopml_parse_file($filename);
- * > print_r($opml_array);
- *
- * > $opml_string = [...];
- * > $opml_array = libopml_parse_string($opml_string);
- * > print_r($opml_array);
- *
- * > $opml_array = [...];
- * > $opml_string = libopml_render($opml_array);
- * > $opml_object = libopml_render($opml_array, true);
- * > echo $opml_string;
- * > print_r($opml_object);
- *
- * You can set $strict argument to false if you want to bypass "text" attribute
- * requirement.
- *
- * If parsing fails for any reason (e.g. not an XML string, does not match with
- * the specifications), a LibOPML_Exception is raised.
- *
- * lib_opml array format is described here:
- * $array = array(
- *     'head' => array(       // 'head' element is optional (but recommended)
- *         'key' => 'value',  // key must be a part of available OPML head elements
- *     ),
- *     'body' => array(              // body is required
- *         array(                    // this array represents an outline (at least one)
- *             'text' => 'value',    // 'text' element is required if $strict is true
- *             'key' => 'value',     // key and value are what you want (optional)
- *             '@outlines' = array(  // @outlines is a special value and represents sub-outlines
- *                 array(
- *                     [...]         // where [...] is a valid outline definition
- *                 ),
- *             ),
- *         ),
- *         array(                    // other outline definitions
- *             [...]
- *         ),
- *         [...],
- *     )
- * )
- *
- */
-
-/**
- * A simple Exception class which represents any kind of OPML problem.
- * Message should precise the current problem.
- */
-class LibOPML_Exception extends Exception {}
-
-
-// Define the list of available head attributes. All of them are optional.
-define('HEAD_ELEMENTS', serialize(array(
-	'title', 'dateCreated', 'dateModified', 'ownerName', 'ownerEmail',
-	'ownerId', 'docs', 'expansionState', 'vertScrollState', 'windowTop',
-	'windowLeft', 'windowBottom', 'windowRight'
-)));
-
-
-/**
- * Parse an XML object as an outline object and return corresponding array
- *
- * @param SimpleXMLElement $outline_xml the XML object we want to parse
- * @param bool $strict true if "text" attribute is required, false else
- * @return array corresponding to an outline and following format described above
- * @throws LibOPML_Exception
- * @access private
- */
-function libopml_parse_outline($outline_xml, $strict = true) {
-	$outline = array();
-
-	// An outline may contain any kind of attributes but "text" attribute is
-	// required !
-	$text_is_present = false;
-
-	$elem = dom_import_simplexml($outline_xml);
-	/** @var DOMAttr $attr */
-	foreach ($elem->attributes as $attr) {
-		$key = $attr->localName;
-
-		if ($attr->namespaceURI == '') {
-			$outline[$key] = $attr->value;
-		} else {
-			$outline[$key] = [
-				'namespace' => $attr->namespaceURI,
-				'value' => $attr->value,
-			];
-		}
-
-		if ($key === 'text') {
-			$text_is_present = true;
-		}
-	}
-
-	if (!$text_is_present && $strict) {
-		throw new LibOPML_Exception(
-			'Outline does not contain any text attribute'
-		);
-	}
-
-	if (empty($outline['text']) && isset($outline['title'])) {
-		$outline['text'] = $outline['title'];
-	}
-
-	foreach ($outline_xml->children() as $key => $value) {
-		// An outline may contain any number of outline children
-		if ($key === 'outline') {
-			$outline['@outlines'][] = libopml_parse_outline($value, $strict);
-		}
-	}
-
-	return $outline;
-}
-
-/**
- * Reformat the XML document as a hierarchy when
- * the OPML 2.0 category attribute is used
- */
-function preprocessing_categories($doc) {
-	$outline_categories = array();
-	$body = $doc->getElementsByTagName('body')->item(0);
-	$xpath = new DOMXpath($doc);
-	$outlines = $xpath->query('/opml/body/outline[@category]');
-	foreach ($outlines as $outline) {
-		$category = trim($outline->getAttribute('category'));
-		if ($category != '') {
-			$outline_category = null;
-			if (!isset($outline_categories[$category])) {
-				$outline_category = $doc->createElement('outline');
-				$outline_category->setAttribute('text', $category);
-				$body->insertBefore($outline_category, $body->firstChild);
-				$outline_categories[$category] = $outline_category;
-			} else {
-				$outline_category = $outline_categories[$category];
-			}
-			$outline->parentNode->removeChild($outline);
-			$outline_category->appendChild($outline);
-		}
-	}
-}
-
-/**
- * Parse a string as a XML one and returns the corresponding array
- *
- * @param string $xml is the string we want to parse
- * @param bool $strict true to perform some validation (e.g. require "text" attribute), false to relax
- * @return array corresponding to the XML string and following format described above
- * @throws LibOPML_Exception
- * @access public
- */
-function libopml_parse_string($xml, $strict = true) {
-	$dom = new DOMDocument();
-	$dom->recover = true;
-	$dom->strictErrorChecking = false;
-	$dom->loadXML($xml);
-	$dom->encoding = 'UTF-8';
-
-	//Partial compatibility with the category attribute of OPML 2.0
-	preprocessing_categories($dom);
-
-	$opml = simplexml_import_dom($dom);
-
-	if (!$opml) {
-		throw new LibOPML_Exception();
-	}
-
-	$array = array(
-		'version' => (string)$opml['version'],
-		'head' => array(),
-		'body' => array()
-	);
-
-	if (isset($opml->head)) {
-		// We get all "head" elements. Head is required but its sub-elements are optional.
-		foreach ($opml->head->children() as $key => $value) {
-			if (in_array($key, unserialize(HEAD_ELEMENTS), true)) {
-				$array['head'][$key] = (string)$value;
-			} elseif ($strict) {
-				throw new LibOPML_Exception($key . ' is not part of the OPML 2.0 specification');
-			}
-		}
-	} elseif ($strict) {
-		throw new LibOPML_Exception('Required OPML head element is missing!');
-	}
-
-	// Then, we get body oulines. Body must contain at least one outline
-	// element.
-	$at_least_one_outline = false;
-	foreach ($opml->body->children() as $key => $value) {
-		if ($key === 'outline') {
-			$at_least_one_outline = true;
-			$array['body'][] = libopml_parse_outline($value, $strict);
-		}
-	}
-
-	if (!$at_least_one_outline) {
-		throw new LibOPML_Exception(
-			'OPML body must contain at least one outline element'
-		);
-	}
-
-	return $array;
-}
-
-
-/**
- * Parse a string contained into a file as a XML string and returns the corresponding array
- *
- * @param string $filename should indicates a valid XML file
- * @param bool $strict true if "text" attribute is required, false else
- * @return array corresponding to the file content and following format described above
- * @throws LibOPML_Exception
- * @access public
- */
-function libopml_parse_file($filename, $strict = true) {
-	$file_content = file_get_contents($filename);
-
-	if ($file_content === false) {
-		throw new LibOPML_Exception(
-			$filename . ' cannot be found'
-		);
-	}
-
-	return libopml_parse_string($file_content, $strict);
-}
-
-
-/**
- * Create a XML outline object in a parent object.
- *
- * @param SimpleXMLElement $parent_elt is the parent object of current outline
- * @param array $outline array representing an outline object
- * @param bool $strict true if "text" attribute is required, false else
- * @throws LibOPML_Exception
- * @access private
- */
-function libopml_render_outline($parent_elt, $outline, $strict) {
-	// Outline MUST be an array!
-	if (!is_array($outline)) {
-		throw new LibOPML_Exception(
-			'Outline element must be defined as array'
-		);
-	}
-
-	$outline_elt = $parent_elt->addChild('outline');
-	$text_is_present = false;
-	/** @var string|array<string,mixed> $value */
-	foreach ($outline as $key => $value) {
-		// Only outlines can be an array and so we consider children are also
-		// outline elements.
-		if ($key === '@outlines' && is_array($value)) {
-			foreach ($value as $outline_child) {
-				libopml_render_outline($outline_elt, $outline_child, $strict);
-			}
-		} elseif (is_array($value) && !isset($value['namespace'])) {
-			throw new LibOPML_Exception(
-				'Type of outline elements cannot be array (except for providing a namespace): ' . $key
-			);
-		} else {
-			// Detect text attribute is present, that's good :)
-			if ($key === 'text') {
-				$text_is_present = true;
-			}
-			if (is_array($value)) {
-				if (!empty($value['namespace']) && !empty($value['value'])) {
-					$outline_elt->addAttribute($key, $value['value'], $value['namespace']);
-				}
-			} else {
-				$outline_elt->addAttribute($key, $value);
-			}
-		}
-	}
-
-	if (!$text_is_present && $strict) {
-		throw new LibOPML_Exception(
-			'You must define at least a text element for all outlines'
-		);
-	}
-}
-
-
-/**
- * Render an array as an OPML string or a XML object.
- *
- * @param array $array is the array we want to render and must follow structure defined above
- * @param bool $as_xml_object false if function must return a string, true for a XML object
- * @param bool $strict true if "text" attribute is required, false else
- * @return string|SimpleXMLElement XML string corresponding to $array or XML object
- * @throws LibOPML_Exception
- * @access public
- */
-function libopml_render($array, $as_xml_object = false, $strict = true) {
-	$opml = new SimpleXMLElement('<opml></opml>');
-	$opml->addAttribute('version', $strict ? '2.0' : '1.0');
-
-	// Create head element. $array['head'] is optional but head element will
-	// exist in the final XML object.
-	$head = $opml->addChild('head');
-	if (isset($array['head'])) {
-		foreach ($array['head'] as $key => $value) {
-			if (in_array($key, unserialize(HEAD_ELEMENTS), true)) {
-				$head->addChild($key, $value);
-			}
-		}
-	}
-
-	// Check body is set and contains at least one element
-	if (!isset($array['body'])) {
-		throw new LibOPML_Exception(
-			'$array must contain a body element'
-		);
-	}
-	if (count($array['body']) <= 0) {
-		throw new LibOPML_Exception(
-			'Body element must contain at least one element (array)'
-		);
-	}
-
-	// Create outline elements
-	$body = $opml->addChild('body');
-	foreach ($array['body'] as $outline) {
-		libopml_render_outline($body, $outline, $strict);
-	}
-
-	// And return the final result
-	if ($as_xml_object) {
-		return $opml;
-	} else {
-		$dom = dom_import_simplexml($opml)->ownerDocument;
-		$dom->formatOutput = true;
-		$dom->encoding = 'UTF-8';
-		return $dom->saveXML();
-	}
-}
diff --git a/lib/lib_rss.php b/lib/lib_rss.php
index cbdfff773..e5362bc5c 100644
--- a/lib/lib_rss.php
+++ b/lib/lib_rss.php
@@ -57,6 +57,11 @@ function classAutoloader($class) {
 		$base_dir = LIB_PATH . '/phpgt/cssxpath/src/';
 		$relative_class_name = substr($class, strlen($prefix));
 		require $base_dir . str_replace('\\', '/', $relative_class_name) . '.php';
+	} elseif (str_starts_with($class, 'marienfressinaud\\LibOpml\\')) {
+		$prefix = 'marienfressinaud\\LibOpml\\';
+		$base_dir = LIB_PATH . '/marienfressinaud/lib_opml/src/LibOpml/';
+		$relative_class_name = substr($class, strlen($prefix));
+		require $base_dir . str_replace('\\', '/', $relative_class_name) . '.php';
 	} elseif (str_starts_with($class, 'PHPMailer\\PHPMailer\\')) {
 		$prefix = 'PHPMailer\\PHPMailer\\';
 		$base_dir = LIB_PATH . '/phpmailer/phpmailer/src/';
diff --git a/lib/marienfressinaud/lib_opml/.gitattributes b/lib/marienfressinaud/lib_opml/.gitattributes
new file mode 100644
index 000000000..669ea8c8d
--- /dev/null
+++ b/lib/marienfressinaud/lib_opml/.gitattributes
@@ -0,0 +1,8 @@
+/.* export-ignore
+
+/ci export-ignore
+/examples export-ignore
+/tests export-ignore
+
+/CHANGELOG.md export-ignore
+/Makefile export-ignore
diff --git a/lib/marienfressinaud/lib_opml/.gitignore b/lib/marienfressinaud/lib_opml/.gitignore
new file mode 100644
index 000000000..ca9baaf91
--- /dev/null
+++ b/lib/marienfressinaud/lib_opml/.gitignore
@@ -0,0 +1,2 @@
+/coverage
+/vendor
diff --git a/lib/marienfressinaud/lib_opml/CHANGELOG.md b/lib/marienfressinaud/lib_opml/CHANGELOG.md
new file mode 100644
index 000000000..ee9245e7e
--- /dev/null
+++ b/lib/marienfressinaud/lib_opml/CHANGELOG.md
@@ -0,0 +1,63 @@
+# Changelog of lib\_opml
+
+## 2022-07-25 - v0.5.0
+
+- BREAKING CHANGE: Reverse parameters in `libopml_render()`
+- BREAKING CHANGE: Validate email and URL address elements
+- Add support for PHP 7.2+
+- Add a .gitattributes file
+- Improve the documentation about usage
+- Add a note about stability in README
+- Fix a PHPDoc annotation
+- Homogeneize tests with "Newspapers" examples
+
+## 2022-06-04 - v0.4.0
+
+- Refactor the LibOpml class to be not static
+- Parse or render attributes according to their types
+- Add support for namespaces
+- Don't require text attribute if OPML version is 1.0
+- Check that outline text attribute is not empty
+- Verify that xmlUrl and url attributes are present according to the type
+  attribute
+- Accept a version attribute in render method
+- Handle OPML 1.1 as 1.0
+- Fail if version, head or body is missing
+- Fail if OPML version is not supported
+- Fail if head contains invalid elements
+- Fail if sub-outlines are not arrays when rendering
+- Make parsing less strict by default
+- Don't raise most parsing errors when strict is false
+- Force type attribute to lowercase
+- Remove SimpleXML as a requirement
+- Homogenize exception messages
+- Close pre tags in the example file
+- Improve documentation in the README
+- Improve comments in the source code
+- Add a MR checklist item about changes
+- Update the description in composer.json
+- Update dev dependencies
+
+## 2022-04-23 - v0.3.0
+
+- Reorganize the architecture of code (using namespaces and classes)
+- Change PHP minimum version to 7.4
+- Move to Framagit instead of GitHub
+- Change the license to MIT
+- Configure lib\_opml with Composer
+- Add PHPUnit tests for all the methods and functions
+- Add a linter to the project
+- Provide a Makefile
+- Configure Gitlab CI instead of Travis
+- Add a merge request template
+- Improve the comments, documentation and examples
+
+## 2014-03-31 - v0.2.0
+
+- Allow to make optional the `text` attribute
+- Improve and complete documentation
+- Fix examples
+
+## 2014-03-29 - v0.1.0
+
+First version
diff --git a/lib/marienfressinaud/lib_opml/LICENSE b/lib/marienfressinaud/lib_opml/LICENSE
new file mode 100644
index 000000000..2ad7f2db4
--- /dev/null
+++ b/lib/marienfressinaud/lib_opml/LICENSE
@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2022 Marien Fressinaud
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
diff --git a/lib/marienfressinaud/lib_opml/README.md b/lib/marienfressinaud/lib_opml/README.md
new file mode 100644
index 000000000..34026bc14
--- /dev/null
+++ b/lib/marienfressinaud/lib_opml/README.md
@@ -0,0 +1,338 @@
+# lib\_opml
+
+lib\_opml is a library to read and write OPML in PHP.
+
+OPML is a standard designed to store and exchange outlines (i.e. a tree
+structure arranged to show hierarchical relationships). It is mainly used to
+exchange list of feeds between feed aggregators. The specification is
+available at [opml.org](http://opml.org).
+
+lib\_opml has been tested with PHP 7.2+. It requires [DOMDocument](https://www.php.net/manual/book.dom.php)
+to work.
+
+It supports versions 1.0 and 2.0 of OPML since these are the only published
+versions. Version 1.1 is treated as version 1.0, as stated by the specification.
+
+It is licensed under the [MIT license](/LICENSE).
+
+## Installation
+
+lib\_opml is available on [Packagist](https://packagist.org/packages/marienfressinaud/lib_opml)
+and it is recommended to install it with Composer:
+
+```console
+$ composer require marienfressinaud/lib_opml
+```
+
+If you don’t use Composer, you can download [the ZIP archive](https://framagit.org/marienfressinaud/lib_opml/-/archive/main/lib_opml-main.zip)
+and copy the content of the `src/` folder in your project. Then, load the files
+manually:
+
+```php
+<?php
+require 'path/to/lib_opml/LibOpml/Exception.php';
+require 'path/to/lib_opml/LibOpml/LibOpml.php';
+require 'path/to/lib_opml/functions.php';
+```
+
+## Usage
+
+### Parse OPML
+
+Let’s say that you have an OPML file named `my_opml_file.xml`:
+
+```xml
+<?xml version="1.0" encoding="UTF-8" ?>
+<opml version="2.0">
+    <head>
+        <title>My OPML</title>
+    </head>
+    <body>
+        <outline text="Newspapers">
+            <outline text="El País" />
+            <outline text="Le Monde" />
+            <outline text="The Guardian" />
+            <outline text="The New York Times" />
+        </outline>
+    </body>
+</opml>
+```
+
+You can load it with:
+
+```php
+$opml_array = libopml_parse_file('my_opml_file.xml');
+```
+
+lib\_opml parses the file and returns an array:
+
+```php
+[
+    'version' => '2.0',
+    'namespaces' => [],
+    'head' => [
+        'title' => 'My OPML'
+    ],
+    'body' => [ // each entry of the body is an outline
+        [
+            'text' => 'Newspapers',
+            '@outlines' => [ // sub-outlines are accessible with the @outlines key
+                ['text' => 'El País'],
+                ['text' => 'Le Monde'],
+                ['text' => 'The Guardian'],
+                ['text' => 'The New York Times']
+            ]
+        ]
+    ]
+]
+```
+
+Since it's just an array, it's very simple to manipulate:
+
+```php
+foreach ($opml_array['body'] as $outline) {
+    echo $outline['text'];
+}
+```
+
+You also can load directly an OPML string:
+
+```php
+$opml_string = '<opml>...</opml>';
+$opml_array = libopml_parse_string($opml_string);
+```
+
+### Render OPML
+
+lib\_opml is able to render an OPML string from an array. It checks that the
+data is valid and respects the specification.
+
+```php
+$opml_array = [
+    'head' => [
+        'title' => 'My OPML',
+    ],
+    'body' => [
+        [
+            'text' => 'Newspapers',
+            '@outlines' => [
+                ['text' => 'El País'],
+                ['text' => 'Le Monde'],
+                ['text' => 'The Guardian'],
+                ['text' => 'The New York Times']
+            ]
+        ]
+    ]
+];
+
+$opml_string = libopml_render($opml_array);
+
+file_put_contents('my_opml_file.xml', $opml_string);
+```
+
+### Handle errors
+
+If rendering (or parsing) fails for any reason (e.g. empty `body`, missing
+`text` attribute, wrong element type), a `\marienfressinaud\LibOpml\Exception`
+is raised:
+
+```php
+try {
+    $opml_array = libopml_render([
+        'body' => []
+    ]);
+} catch (\marienfressinaud\LibOpml\Exception $e) {
+    echo $e->getMessage();
+}
+```
+
+### Class style
+
+lib\_opml can also be used with a class style:
+
+```php
+use marienfressinaud\LibOpml;
+
+$libopml = new LibOpml\LibOpml();
+
+$opml_array = $libopml->parseFile($filename);
+$opml_array = $libopml->parseString($opml_string);
+$opml_string = $libopml->render($opml_array);
+```
+
+### Special elements and attributes
+
+Some elements have special meanings according to the specification, which means
+they can be parsed to a specific type by lib\_opml. In the other way, when
+rendering an OPML string, you must pass these elements with their correct
+types.
+
+Head elements:
+
+- `dateCreated` is parsed to a `\DateTime`;
+- `dateModified` is parsed to a `\DateTime`;
+- `expansionState` is parsed to an array of integers;
+- `vertScrollState` is parsed to an integer;
+- `windowTop` is parsed to an integer;
+- `windowLeft` is parsed to an integer;
+- `windowBottom` is parsed to an integer;
+- `windowRight` is parsed to an integer.
+
+Outline attributes:
+
+- `created` is parsed to a `\DateTime`;
+- `category` is parsed to an array of strings;
+- `isComment` is parsed to a boolean;
+- `isBreakpoint` is parsed to a boolean.
+
+If one of these elements is not of the correct type, an Exception is raised.
+
+Finally, there are additional checks based on the outline type attribute:
+
+- if `type="rss"`, then the `xmlUrl` attribute is required;
+- if `type="link"`, then the `url` attribute is required;
+- if `type="include"`, then the `url` attribute is required.
+
+Note that the `type` attribute is case-insensitive and will always be lowercased.
+
+### Namespaces
+
+OPML can be extended with namespaces:
+
+> An OPML file may contain elements and attributes not described on this page,
+> only if those elements are defined in a namespace, as specified by the W3C.
+
+When rendering an OPML, you can include a `namespaces` key to specify
+namespaces:
+
+```php
+$opml_array = [
+    'namespaces' => [
+        'test' => 'https://example.com/test',
+    ],
+    'body' => [
+        ['text' => 'My outline', 'test:path' => '/some/example/path'],
+    ],
+];
+
+$opml_string = libopml_render($opml_array);
+echo $opml_string;
+```
+
+This will output:
+
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
+<opml xmlns:test="https://example.com/test" version="2.0">
+  <head/>
+  <body>
+    <outline text="My outline" test:path="/some/example/path"/>
+  </body>
+</opml>
+```
+
+### Strictness
+
+You can tell lib\_opml to be less or more strict when parsing or rendering OPML.
+This is done by passing an optional `$strict` attribute to the functions. When
+strict is `false`, most of the specification requirements are simply ignored
+and lib\_opml will do its best to parse (or generate) an OPML.
+
+By default, parsing is not strict so you’ll be able to read most of the files
+out there. If you want the parsing to be strict (to validate a file for
+instance), pass `true` to `libopml_parse_file()` or `libopml_parse_string()`:
+
+```php
+$opml_array = libopml_parse_file($filename, true);
+$opml_array = libopml_parse_string($opml_string, true);
+```
+
+On the other side, reading is strict by default, so you are encouraged to
+generate valid OPMLs. If you need to relax the strictness, pass `false` to
+`libopml_render()`:
+
+```php
+$opml_string = libopml_render($opml_array, false);
+```
+
+Please note that when using the class form, strict is passed during the object
+instantiation:
+
+```php
+use marienfressinaud\LibOpml;
+
+// lib_opml will be strict for both parsing and rendering!
+$libopml = new LibOpml\LibOpml(true);
+
+$opml_array = $libopml->parseString($opml_string);
+$opml_string = $libopml->render($opml_array);
+```
+
+## Examples and documented source code
+
+See the [`examples/`](/examples) folder for concrete examples.
+
+You are encouraged to read the source code to learn more about lib\_opml. Thus,
+the full documentation is available as comments in the code:
+
+- [`src/LibOpml/LibOpml.php`](src/LibOpml/LibOpml.php)
+- [`src/LibOpml/Exception.php`](src/LibOpml/Exception.php)
+- [`src/functions.php`](src/functions.php)
+
+## Changelog
+
+See [CHANGELOG.md](/CHANGELOG.md).
+
+## Support and stability
+
+Today, lib\_opml covers all the aspects of the OPML specification. Since the
+spec didn't change for more than 15 years, it is expected for the library to
+not change a lot in the future. Thus, I plan to release the v1.0 in a near
+future. I'm only waiting for more tests to be done on its latest version (in
+particular in FreshRSS, see [FreshRSS/FreshRSS#4403](https://github.com/FreshRSS/FreshRSS/pull/4403)).
+I would also wait for clarifications about the specification (see [scripting/opml.org#3](https://github.com/scripting/opml.org/issues/3)),
+but it isn't a hard requirement.
+
+After the release of 1.0, lib\_opml will be considered as “finished”. This
+means I will not add new features, nor break the existing code. However, I
+commit myself to continue to support the library to fix security issues, bugs,
+or to add support to new PHP versions.
+
+In consequence, you can expect lib\_opml to be stable.
+
+## Tests and linters
+
+This section is for developers of lib\_opml.
+
+To run the tests, you’ll have to install Composer first (see [the official
+documentation](https://getcomposer.org/doc/00-intro.md)). Then, install the
+dependencies:
+
+```console
+$ make install
+```
+
+You should now have a `vendor/` folder containing the development dependencies.
+
+Run the tests with:
+
+```console
+$ make test
+```
+
+Run the linter with:
+
+```console
+$ make lint
+$ make lint-fix
+```
+
+## Contributing
+
+Please submit bug reports and merge requests to the [Framagit repository](https://framagit.org/marienfressinaud/lib_opml).
+
+There’s not a lot to do, but the documentation and examples could probably be
+improved.
+
+Merge requests require that you fill a short checklist to save me time while
+reviewing your changes. You also must make sure the test suite succeeds.
diff --git a/lib/marienfressinaud/lib_opml/composer.json b/lib/marienfressinaud/lib_opml/composer.json
new file mode 100644
index 000000000..ba48d16ed
--- /dev/null
+++ b/lib/marienfressinaud/lib_opml/composer.json
@@ -0,0 +1,35 @@
+{
+    "name": "marienfressinaud/lib_opml",
+    "description": "A library to read and write OPML in PHP.",
+    "license": "MIT",
+    "authors": [
+        {
+            "name": "Marien Fressinaud",
+            "email": "dev@marienfressinaud.fr"
+        }
+    ],
+    "require": {
+        "php": ">=7.2.0",
+        "ext-dom": "*"
+    },
+    "config": {
+        "platform": {
+            "php": "7.2.0"
+        }
+    },
+    "support": {
+        "issues": "https://framagit.org/marienfressinaud/lib_opml/-/issues"
+    },
+    "autoload": {
+        "files": [
+            "src/functions.php"
+        ],
+        "psr-4": {
+            "marienfressinaud\\": "src/"
+        }
+    },
+    "require-dev": {
+        "squizlabs/php_codesniffer": "^3.6",
+        "phpunit/phpunit": "^8"
+    }
+}
diff --git a/lib/marienfressinaud/lib_opml/src/LibOpml/Exception.php b/lib/marienfressinaud/lib_opml/src/LibOpml/Exception.php
new file mode 100644
index 000000000..27c3287a2
--- /dev/null
+++ b/lib/marienfressinaud/lib_opml/src/LibOpml/Exception.php
@@ -0,0 +1,15 @@
+<?php
+
+namespace marienfressinaud\LibOpml;
+
+/**
+ * A simple Exception class which represents any kind of OPML problem.
+ * Message precises the current problem.
+ *
+ * @author Marien Fressinaud <dev@marienfressinaud.fr>
+ * @link https://framagit.org/marienfressinaud/lib_opml
+ * @license MIT
+ */
+class Exception extends \Exception
+{
+}
diff --git a/lib/marienfressinaud/lib_opml/src/LibOpml/LibOpml.php b/lib/marienfressinaud/lib_opml/src/LibOpml/LibOpml.php
new file mode 100644
index 000000000..4ba0df821
--- /dev/null
+++ b/lib/marienfressinaud/lib_opml/src/LibOpml/LibOpml.php
@@ -0,0 +1,770 @@
+<?php
+
+namespace marienfressinaud\LibOpml;
+
+/**
+ * The LibOpml class provides the methods to read and write OPML files and
+ * strings. It transforms OPML files or strings to PHP arrays (or the reverse).
+ *
+ * How to read this file?
+ *
+ * The first methods are dedicated to the parsing, and the next ones to the
+ * reading. The three last methods are helpful methods, but you don't have to
+ * worry too much about them.
+ *
+ * The main methods are the public ones: parseFile, parseString and render.
+ * They call the other parse* and render* methods internally.
+ *
+ * These three main methods are available as functions (see the src/functions.php
+ * file).
+ *
+ * What's the array format?
+ *
+ * As said before, LibOpml transforms OPML to PHP arrays, or the reverse. The
+ * format is pretty simple. It contains four keys:
+ *
+ * - version: the version of the OPML;
+ * - namespaces: an array of namespaces used in the OPML, if any;
+ * - head: an array of OPML head elements, where keys are the names of the
+ *   elements;
+ * - body: an array of arrays representing OPML outlines, where keys are the
+ *   name of the attributes (the special @outlines key contains the sub-outlines).
+ *
+ * When rendering, only the body key is required (version will default to 2.0).
+ *
+ * Example:
+ *
+ * [
+ *     version => '2.0',
+ *     namespaces => [],
+ *     head => [
+ *         title => 'An OPML file'
+ *     ],
+ *     body => [
+ *         [
+ *             text => 'Newspapers',
+ *             @outlines => [
+ *                 [text => 'El País'],
+ *                 [text => 'Le Monde'],
+ *                 [text => 'The Guardian'],
+ *                 [text => 'The New York Times'],
+ *             ]
+ *         ]
+ *     ]
+ * ]
+ *
+ * @see http://opml.org/spec2.opml
+ *
+ * @author Marien Fressinaud <dev@marienfressinaud.fr>
+ * @link https://framagit.org/marienfressinaud/lib_opml
+ * @license MIT
+ */
+class LibOpml
+{
+    /**
+     * The list of valid head elements.
+     */
+    public const HEAD_ELEMENTS = [
+        'title', 'dateCreated', 'dateModified', 'ownerName', 'ownerEmail',
+        'ownerId', 'docs', 'expansionState', 'vertScrollState', 'windowTop',
+        'windowLeft', 'windowBottom', 'windowRight'
+    ];
+
+    /**
+     * The list of numeric head elements.
+     */
+    public const NUMERIC_HEAD_ELEMENTS = [
+        'vertScrollState',
+        'windowTop',
+        'windowLeft',
+        'windowBottom',
+        'windowRight',
+    ];
+
+    /** @var boolean */
+    private $strict = true;
+
+    /** @var string */
+    private $version = '2.0';
+
+    /** @var string[] */
+    private $namespaces = [];
+
+    /**
+     * @param bool $strict
+     *     Set to true (default) to check for violations of the specification,
+     *     false otherwise.
+     */
+    public function __construct($strict = true)
+    {
+        $this->strict = $strict;
+    }
+
+    /**
+     * Parse a XML file and return the corresponding array.
+     *
+     * @param string $filename
+     *     The XML file to parse.
+     *
+     * @throws \marienfressinaud\LibOpml\Exception
+     *     Raised if the file cannot be read. See also exceptions raised by the
+     *     parseString method.
+     *
+     * @return array
+     *     An array reflecting the OPML (the structure is described above).
+     */
+    public function parseFile($filename)
+    {
+        $file_content = @file_get_contents($filename);
+
+        if ($file_content === false) {
+            throw new Exception("OPML file {$filename} cannot be found or read");
+        }
+
+        return $this->parseString($file_content);
+    }
+
+    /**
+     * Parse a XML string and return the corresponding array.
+     *
+     * @param string $xml
+     *     The XML string to parse.
+     *
+     * @throws \marienfressinaud\LibOpml\Exception
+     *     Raised if the XML cannot be parsed, if version is missing or
+     *     invalid, if head is missing or contains invalid (or not parsable)
+     *     elements, or if body is missing, empty or contain non outline
+     *     elements. The exceptions (except XML parsing errors) are not raised
+     *     if strict is false. See also exceptions raised by the parseOutline
+     *     method.
+     *
+     * @return array
+     *     An array reflecting the OPML (the structure is described above).
+     */
+    public function parseString($xml)
+    {
+        $dom = new \DOMDocument();
+        $dom->recover = true;
+        $dom->encoding = 'UTF-8';
+
+        try {
+            $result = @$dom->loadXML($xml);
+        } catch (\Exception | \Error $e) {
+            $result = false;
+        }
+
+        if (!$result) {
+            throw new Exception('OPML string is not valid XML');
+        }
+
+        $opml_element = $dom->documentElement;
+
+        // Load the custom namespaces of the document
+        $xpath = new \DOMXPath($dom);
+        $this->namespaces = [];
+        foreach ($xpath->query('//namespace::*') as $node) {
+            if ($node->prefix === 'xml') {
+                // This is the base namespace, we don't need to store it
+                continue;
+            }
+
+            $this->namespaces[$node->prefix] = $node->namespaceURI;
+        }
+
+        // Get the version of the document
+        $version = $opml_element->getAttribute('version');
+        if (!$version) {
+            $this->throwExceptionIfStrict('OPML version attribute is required');
+        }
+
+        $version = trim($version);
+        if ($version === '1.1') {
+            $version = '1.0';
+        }
+
+        if ($version !== '1.0' && $version !== '2.0') {
+            $this->throwExceptionIfStrict('OPML supported versions are 1.0 and 2.0');
+        }
+
+        $this->version = $version;
+
+        // Get head and body child elements
+        $head_elements = $opml_element->getElementsByTagName('head');
+        $child_head_elements = [];
+        if (count($head_elements) === 1) {
+            $child_head_elements = $head_elements[0]->childNodes;
+        } else {
+            $this->throwExceptionIfStrict('OPML must contain one and only one head element');
+        }
+
+        $body_elements = $opml_element->getElementsByTagName('body');
+        $child_body_elements = [];
+        if (count($body_elements) === 1) {
+            $child_body_elements = $body_elements[0]->childNodes;
+        } else {
+            $this->throwExceptionIfStrict('OPML must contain one and only one body element');
+        }
+
+        $array = [
+            'version' => $this->version,
+            'namespaces' => $this->namespaces,
+            'head' => [],
+            'body' => [],
+        ];
+
+        // Load the child head elements in the head array
+        foreach ($child_head_elements as $child_head_element) {
+            if ($child_head_element->nodeType !== XML_ELEMENT_NODE) {
+                continue;
+            }
+
+            $name = $child_head_element->nodeName;
+            $value = $child_head_element->nodeValue;
+            $namespaced = $child_head_element->namespaceURI !== null;
+
+            if (!in_array($name, self::HEAD_ELEMENTS) && !$namespaced) {
+                $this->throwExceptionIfStrict(
+                    "OPML head {$name} element is not part of the specification"
+                );
+            }
+
+            if ($name === 'dateCreated' || $name === 'dateModified') {
+                try {
+                    $value = $this->parseDate($value);
+                } catch (\DomainException $e) {
+                    $this->throwExceptionIfStrict(
+                        "OPML head {$name} element must be a valid RFC822 or RFC1123 date"
+                    );
+                }
+            } elseif ($name === 'ownerEmail') {
+                // Testing email validity is hard. PHP filter_var() function is
+                // too strict compared to the RFC 822, so we can't use it.
+                if (strpos($value, '@') === false) {
+                    $this->throwExceptionIfStrict(
+                        'OPML head ownerEmail element must be an email address'
+                    );
+                }
+            } elseif ($name === 'ownerId' || $name === 'docs') {
+                if (!$this->checkHttpAddress($value)) {
+                    $this->throwExceptionIfStrict(
+                        "OPML head {$name} element must be a HTTP address"
+                    );
+                }
+            } elseif ($name === 'expansionState') {
+                $numbers = explode(',', $value);
+                $value = array_map(function ($str_number) {
+                    if (is_numeric($str_number)) {
+                        return intval($str_number);
+                    } else {
+                        $this->throwExceptionIfStrict(
+                            'OPML head expansionState element must be a list of numbers'
+                        );
+                        return $str_number;
+                    }
+                }, $numbers);
+            } elseif (in_array($name, self::NUMERIC_HEAD_ELEMENTS)) {
+                if (is_numeric($value)) {
+                    $value = intval($value);
+                } else {
+                    $this->throwExceptionIfStrict("OPML head {$name} element must be a number");
+                }
+            }
+
+            $array['head'][$name] = $value;
+        }
+
+        // Load the child body elements in the body array
+        foreach ($child_body_elements as $child_body_element) {
+            if ($child_body_element->nodeType !== XML_ELEMENT_NODE) {
+                continue;
+            }
+
+            if ($child_body_element->nodeName === 'outline') {
+                $array['body'][] = $this->parseOutline($child_body_element);
+            } else {
+                $this->throwExceptionIfStrict(
+                    'OPML body element can only contain outline elements'
+                );
+            }
+        }
+
+        if (empty($array['body'])) {
+            $this->throwExceptionIfStrict(
+                'OPML body element must contain at least one outline element'
+            );
+        }
+
+        return $array;
+    }
+
+    /**
+     * Parse a XML element as an outline element and return the corresponding array.
+     *
+     * @param \DOMElement $outline_element
+     *     The element to parse.
+     *
+     * @throws \marienfressinaud\LibOpml\Exception
+     *     Raised if the outline contains non-outline elements, if it doesn't
+     *     contain a text attribute (or if empty), if a special attribute is
+     *     not parsable, or if type attribute requirements are not met. The
+     *     exceptions are not raised if strict is false. The exception about
+     *     missing text attribute is not raised if version is 1.0.
+     *
+     * @return array
+     *     An array reflecting the OPML outline (the structure is described above).
+     */
+    private function parseOutline($outline_element)
+    {
+        $outline = [];
+
+        // Load the element attributes in the outline array
+        foreach ($outline_element->attributes as $outline_attribute) {
+            $name = $outline_attribute->nodeName;
+            $value = $outline_attribute->nodeValue;
+
+            if ($name === 'created') {
+                try {
+                    $value = $this->parseDate($value);
+                } catch (\DomainException $e) {
+                    $this->throwExceptionIfStrict(
+                        'OPML outline created attribute must be a valid RFC822 or RFC1123 date'
+                    );
+                }
+            } elseif ($name === 'category') {
+                $categories = explode(',', $value);
+                $categories = array_map(function ($category) {
+                    return trim($category);
+                }, $categories);
+                $value = $categories;
+            } elseif ($name === 'isComment' || $name === 'isBreakpoint') {
+                if ($value === 'true' || $value === 'false') {
+                    $value = $value === 'true';
+                } else {
+                    $this->throwExceptionIfStrict(
+                        "OPML outline {$name} attribute must be a boolean (true or false)"
+                    );
+                }
+            } elseif ($name === 'type') {
+                // type attribute is case-insensitive
+                $value = strtolower($value);
+            }
+
+            $outline[$name] = $value;
+        }
+
+        if (empty($outline['text']) && $this->version !== '1.0') {
+            $this->throwExceptionIfStrict(
+                'OPML outline text attribute is required'
+            );
+        }
+
+        // Perform additional check based on the type of the outline
+        $type = $outline['type'] ?? '';
+        if ($type === 'rss') {
+            if (empty($outline['xmlUrl'])) {
+                $this->throwExceptionIfStrict(
+                    'OPML outline xmlUrl attribute is required when type is "rss"'
+                );
+            } elseif (!$this->checkHttpAddress($outline['xmlUrl'])) {
+                $this->throwExceptionIfStrict(
+                    'OPML outline xmlUrl attribute must be a HTTP address when type is "rss"'
+                );
+            }
+        } elseif ($type === 'link' || $type === 'include') {
+            if (empty($outline['url'])) {
+                $this->throwExceptionIfStrict(
+                    "OPML outline url attribute is required when type is \"{$type}\""
+                );
+            } elseif (!$this->checkHttpAddress($outline['url'])) {
+                $this->throwExceptionIfStrict(
+                    "OPML outline url attribute must be a HTTP address when type is \"{$type}\""
+                );
+            }
+        }
+
+        // Load the sub-outlines in a @outlines array
+        foreach ($outline_element->childNodes as $child_outline_element) {
+            if ($child_outline_element->nodeType !== XML_ELEMENT_NODE) {
+                continue;
+            }
+
+            if ($child_outline_element->nodeName === 'outline') {
+                $outline['@outlines'][] = $this->parseOutline($child_outline_element);
+            } else {
+                $this->throwExceptionIfStrict(
+                    'OPML body element can only contain outline elements'
+                );
+            }
+        }
+
+        return $outline;
+    }
+
+    /**
+     * Parse a value as a date.
+     *
+     * @param string $value
+     *
+     * @throws \DomainException
+     *     Raised if the value cannot be parsed.
+     *
+     * @return \DateTime
+     */
+    private function parseDate($value)
+    {
+        $formats = [
+            \DateTimeInterface::RFC822,
+            \DateTimeInterface::RFC1123,
+        ];
+
+        foreach ($formats as $format) {
+            $date = date_create_from_format($format, $value);
+            if ($date !== false) {
+                return $date;
+            }
+        }
+
+        throw new \DomainException('The argument cannot be parsed as a date');
+    }
+
+    /**
+     * Render an OPML array as a string or a \DOMDocument.
+     *
+     * @param array $array
+     *     The array to render, it must follow the structure defined above.
+     * @param bool $as_dom_document
+     *     Set to false (default) to return the array as a string, true to
+     *     return as a \DOMDocument.
+     *
+     * @throws \marienfressinaud\LibOpml\Exception
+     *     Raised if the `head` array contains unknown or invalid elements
+     *     (i.e. not of correct type), or if the `body` array is missing or
+     *     empty. The exceptions are not raised if strict is false. See also
+     *     exceptions raised by the renderOutline method.
+     *
+     * @return string|\DOMDocument
+     *     The XML string or DOM document corresponding to the given array.
+     */
+    public function render($array, $as_dom_document = false)
+    {
+        $dom = new \DOMDocument('1.0', 'UTF-8');
+        $opml_element = new \DOMElement('opml');
+        $dom->appendChild($opml_element);
+
+        // Set the version attribute of the OPML document
+        $version = $array['version'] ?? '2.0';
+
+        if ($version === '1.1') {
+            $version = '1.0';
+        }
+
+        if ($version !== '1.0' && $version !== '2.0') {
+            $this->throwExceptionIfStrict('OPML supported versions are 1.0 and 2.0');
+        }
+
+        $this->version = $version;
+        $opml_element->setAttribute('version', $this->version);
+
+        // Declare the namespace on the opml element
+        $this->namespaces = $array['namespaces'] ?? [];
+        foreach ($this->namespaces as $prefix => $namespace) {
+            $opml_element->setAttributeNS(
+                'http://www.w3.org/2000/xmlns/',
+                "xmlns:{$prefix}",
+                $namespace
+            );
+        }
+
+        // Add the head element to the OPML document. $array['head'] is
+        // optional but head tag will always exist in the final XML.
+        $head_element = new \DOMElement('head');
+        $opml_element->appendChild($head_element);
+        if (isset($array['head'])) {
+            foreach ($array['head'] as $name => $value) {
+                $namespace = $this->getNamespace($name);
+
+                if (!in_array($name, self::HEAD_ELEMENTS, true) && !$namespace) {
+                    $this->throwExceptionIfStrict(
+                        "OPML head {$name} element is not part of the specification"
+                    );
+                }
+
+                if ($name === 'dateCreated' || $name === 'dateModified') {
+                    if ($value instanceof \DateTimeInterface) {
+                        $value = $value->format(\DateTimeInterface::RFC1123);
+                    } else {
+                        $this->throwExceptionIfStrict(
+                            "OPML head {$name} element must be a DateTime"
+                        );
+                    }
+                } elseif ($name === 'ownerEmail') {
+                    // Testing email validity is hard. PHP filter_var() function is
+                    // too strict compared to the RFC 822, so we can't use it.
+                    if (strpos($value, '@') === false) {
+                        $this->throwExceptionIfStrict(
+                            'OPML head ownerEmail element must be an email address'
+                        );
+                    }
+                } elseif ($name === 'ownerId' || $name === 'docs') {
+                    if (!$this->checkHttpAddress($value)) {
+                        $this->throwExceptionIfStrict(
+                            "OPML head {$name} element must be a HTTP address"
+                        );
+                    }
+                } elseif ($name === 'expansionState') {
+                    if (is_array($value)) {
+                        foreach ($value as $number) {
+                            if (!is_int($number)) {
+                                $this->throwExceptionIfStrict(
+                                    'OPML head expansionState element must be an array of integers'
+                                );
+                            }
+                        }
+
+                        $value = implode(', ', $value);
+                    } else {
+                        $this->throwExceptionIfStrict(
+                            'OPML head expansionState element must be an array of integers'
+                        );
+                    }
+                } elseif (in_array($name, self::NUMERIC_HEAD_ELEMENTS)) {
+                    if (!is_int($value)) {
+                        $this->throwExceptionIfStrict(
+                            "OPML head {$name} element must be an integer"
+                        );
+                    }
+                }
+
+                $child_head_element = new \DOMElement($name, $value, $namespace);
+                $head_element->appendChild($child_head_element);
+            }
+        }
+
+        // Check body is set and contains at least one element
+        if (!isset($array['body'])) {
+            $this->throwExceptionIfStrict('OPML array must contain a body key');
+        }
+
+        $array_body = $array['body'] ?? [];
+        if (count($array_body) <= 0) {
+            $this->throwExceptionIfStrict(
+                'OPML body element must contain at least one outline array'
+            );
+        }
+
+        // Create outline elements in the body element
+        $body_element = new \DOMElement('body');
+        $opml_element->appendChild($body_element);
+        foreach ($array_body as $outline) {
+            $this->renderOutline($body_element, $outline);
+        }
+
+        // And return the final result
+        if ($as_dom_document) {
+            return $dom;
+        } else {
+            $dom->formatOutput = true;
+            return $dom->saveXML();
+        }
+    }
+
+    /**
+     * Transform an outline array to a \DOMElement and add it to a parent element.
+     *
+     * @param \DOMElement $parent_element
+     *     The DOM parent element of the current outline.
+     * @param array $outline
+     *     The outline array to transform in a \DOMElement, it must follow the
+     *     structure defined above.
+     *
+     * @throws \marienfressinaud\LibOpml\Exception
+     *     Raised if the outline is not an array, if it doesn't contain a text
+     *     attribute (or if empty), if the `@outlines` key is not an array, if
+     *     a special attribute does not match its corresponding type, or if
+     *     `type` key requirements are not met. The exceptions (except errors
+     *     about outline or suboutlines not being arrays) are not raised if
+     *     strict is false. The exception about missing text attribute is not
+     *     raised if version is 1.0.
+     */
+    private function renderOutline($parent_element, $outline)
+    {
+        // Perform initial checks to verify the outline is correctly declared
+        if (!is_array($outline)) {
+            throw new Exception(
+                'OPML outline element must be defined as an array'
+            );
+        }
+
+        if (empty($outline['text']) && $this->version !== '1.0') {
+            $this->throwExceptionIfStrict(
+                'OPML outline text attribute is required'
+            );
+        }
+
+        if (isset($outline['type'])) {
+            $type = strtolower($outline['type']);
+
+            if ($type === 'rss') {
+                if (empty($outline['xmlUrl'])) {
+                    $this->throwExceptionIfStrict(
+                        'OPML outline xmlUrl attribute is required when type is "rss"'
+                    );
+                } elseif (!$this->checkHttpAddress($outline['xmlUrl'])) {
+                    $this->throwExceptionIfStrict(
+                        'OPML outline xmlUrl attribute must be a HTTP address when type is "rss"'
+                    );
+                }
+            } elseif ($type === 'link' || $type === 'include') {
+                if (empty($outline['url'])) {
+                    $this->throwExceptionIfStrict(
+                        "OPML outline url attribute is required when type is \"{$type}\""
+                    );
+                } elseif (!$this->checkHttpAddress($outline['url'])) {
+                    $this->throwExceptionIfStrict(
+                        "OPML outline url attribute must be a HTTP address when type is \"{$type}\""
+                    );
+                }
+            }
+        }
+
+        // Create the outline element and add it to the parent
+        $outline_element = new \DOMElement('outline');
+        $parent_element->appendChild($outline_element);
+
+        // Load the sub-outlines as child elements
+        if (isset($outline['@outlines'])) {
+            $outline_children = $outline['@outlines'];
+
+            if (!is_array($outline_children)) {
+                throw new Exception(
+                    'OPML outline element must be defined as an array'
+                );
+            }
+
+            foreach ($outline_children as $outline_child) {
+                $this->renderOutline($outline_element, $outline_child);
+            }
+
+            // We don't want the sub-outlines to be loaded as attributes, so we
+            // remove the key from the array.
+            unset($outline['@outlines']);
+        }
+
+        // Load the other elements of the array as attributes
+        foreach ($outline as $name => $value) {
+            $namespace = $this->getNamespace($name);
+
+            if ($name === 'created') {
+                if ($value instanceof \DateTimeInterface) {
+                    $value = $value->format(\DateTimeInterface::RFC1123);
+                } else {
+                    $this->throwExceptionIfStrict(
+                        'OPML outline created attribute must be a DateTime'
+                    );
+                }
+            } elseif ($name === 'isComment' || $name === 'isBreakpoint') {
+                if (is_bool($value)) {
+                    $value = $value ? 'true' : 'false';
+                } else {
+                    $this->throwExceptionIfStrict(
+                        "OPML outline {$name} attribute must be a boolean"
+                    );
+                }
+            } elseif (is_array($value)) {
+                $value = implode(', ', $value);
+            }
+
+            $outline_element->setAttributeNS($namespace, $name, $value);
+        }
+    }
+
+    /**
+     * Return wether a value is a valid HTTP address or not.
+     *
+     * HTTP address is not strictly defined by the OPML spec, so it is assumed:
+     *
+     * - it can be parsed by parse_url
+     * - it has a host part
+     * - scheme is http or https
+     *
+     * filter_var is not used because it would reject internationalized URLs
+     * (i.e. with non ASCII chars). An alternative would be to punycode such
+     * URLs, but it's more work to do it properly, and lib_opml needs to stay
+     * simple.
+     *
+     * @param string $value
+     *
+     * @return boolean
+     *     Return true if the value is a valid HTTP address, false otherwise.
+     */
+    public function checkHttpAddress($value)
+    {
+        $value = trim($value);
+        $parsed_url = parse_url($value);
+        if (!$parsed_url) {
+            return false;
+        }
+
+        if (
+            !isset($parsed_url['scheme']) ||
+            !isset($parsed_url['host'])
+        ) {
+            return false;
+        }
+
+        if (
+            $parsed_url['scheme'] !== 'http' &&
+            $parsed_url['scheme'] !== 'https'
+        ) {
+            return false;
+        }
+
+        return true;
+    }
+
+    /**
+     * Return the namespace of a qualified name. An empty string is returned if
+     * the name is not namespaced.
+     *
+     * @param string $qualified_name
+     *
+     * @throws \marienfressinaud\LibOpml\Exception
+     *     Raised if the namespace prefix isn't declared.
+     *
+     * @return string
+     */
+    private function getNamespace($qualified_name)
+    {
+        $split_name = explode(':', $qualified_name, 2);
+        // count will always be 1 or 2.
+        if (count($split_name) === 1) {
+            // If 1, there's no prefix, thus no namespace
+            return '';
+        } else {
+            // If 2, it means it has a namespace prefix, so we get the
+            // namespace from the declared ones.
+            $namespace_prefix = $split_name[0];
+            if (!isset($this->namespaces[$namespace_prefix])) {
+                throw new Exception(
+                    "OPML namespace {$namespace_prefix} is not declared"
+                );
+            }
+
+            return $this->namespaces[$namespace_prefix];
+        }
+    }
+
+    /**
+     * Raise an exception only if strict is true.
+     *
+     * @param string $message
+     *
+     * @throws \marienfressinaud\LibOpml\Exception
+     */
+    private function throwExceptionIfStrict($message)
+    {
+        if ($this->strict) {
+            throw new Exception($message);
+        }
+    }
+}