Automated test towards PHPStan level 6 (#5230)

* Automated test towards PHPStan level 6
Contributes to https://github.com/FreshRSS/FreshRSS/issues/4112
With this automated tests, files that are passing Level 6 are tested to avoid regressions towards Level 5 (which is the current baseline for the whole code-base)

It is included when calling `make test-all` or can be called specifically by `composer run-script phpstan-next`

* Test and example with 3 new passing files

7 files changed, 105 insertions, 5 deletions

diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index 6e1e6b0a6..ce0be20ce 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -49,6 +49,9 @@ jobs:
     - name: PHPStan
       run: composer run-script phpstan
 
+    - name: PHPStan Next Level
+      run: composer run-script phpstan-next
+
     # NPM tests
 
     - name: Uses Node.js
diff --git a/app/Models/SystemConfiguration.php b/app/Models/SystemConfiguration.php
index 9fc79969d..2eed1f02a 100644
--- a/app/Models/SystemConfiguration.php
+++ b/app/Models/SystemConfiguration.php
@@ -27,7 +27,7 @@
  */
 final class FreshRSS_SystemConfiguration extends Minz_Configuration {
 
-	public static function init($config_filename, $default_filename = null): FreshRSS_SystemConfiguration {
+	public static function init(string $config_filename, ?string $default_filename = null): FreshRSS_SystemConfiguration {
 		parent::register('system', $config_filename, $default_filename);
 		return parent::get('system');
 	}
diff --git a/app/Models/UserConfiguration.php b/app/Models/UserConfiguration.php
index f539268ae..77e1d2821 100644
--- a/app/Models/UserConfiguration.php
+++ b/app/Models/UserConfiguration.php
@@ -71,7 +71,8 @@
  */
 final class FreshRSS_UserConfiguration extends Minz_Configuration {
 
-	public static function init($config_filename, $default_filename = null, $configuration_setter = null): FreshRSS_UserConfiguration {
+	public static function init(string $config_filename, ?string $default_filename = null,
+		?FreshRSS_ConfigurationSetter $configuration_setter = null): FreshRSS_UserConfiguration {
 		parent::register('user', $config_filename, $default_filename, $configuration_setter);
 		return parent::get('user');
 	}
diff --git a/app/actualize_script.php b/app/actualize_script.php
index bad466e64..8e445b1e7 100755
--- a/app/actualize_script.php
+++ b/app/actualize_script.php
@@ -25,7 +25,7 @@ define('SIMPLEPIE_SYSLOG_ENABLED', FreshRSS_Context::$system_conf->simplepie_sys
  * Writes to FreshRSS admin log, and if it is not already done by default,
  * writes to syslog (only if simplepie_syslog_enabled in FreshRSS configuration) and to STDOUT
  */
-function notice($message) {
+function notice(string $message): void {
 	Minz_Log::notice($message, ADMIN_LOG);
 	// @phpstan-ignore-next-line
 	if (!COPY_LOG_TO_SYSLOG && SIMPLEPIE_SYSLOG_ENABLED) {
diff --git a/app/views/helpers/export/opml.phtml b/app/views/helpers/export/opml.phtml
index 64c83c960..64dc18eba 100644
--- a/app/views/helpers/export/opml.phtml
+++ b/app/views/helpers/export/opml.phtml
@@ -2,8 +2,9 @@
 
 /**
  * @param array<FreshRSS_Feed> $feeds
+ * @return array<array<string,string|null>>
  */
-function feedsToOutlines($feeds, $excludeMutedFeeds = false): array {
+function feedsToOutlines($feeds, bool $excludeMutedFeeds = false): array {
 	$outlines = [];
 	foreach ($feeds as $feed) {
 		if ($feed->mute() && $excludeMutedFeeds) {
diff --git a/composer.json b/composer.json
index 2234a0e13..461c2f457 100644
--- a/composer.json
+++ b/composer.json
@@ -57,6 +57,7 @@
         "phpcs": "phpcs . -s",
         "phpcbf": "phpcbf . -p -s",
         "phpstan": "phpstan analyse --memory-limit 512M .",
+        "phpstan-next": "phpstan analyse --level 6 --memory-limit 512M $(find . -type d -name 'vendor' -prune -o -name '*.php' -o -name '*.phtml' | grep -Fxvf ./tests/phpstan-next.txt | sort | paste -s)",
         "phpunit": "phpunit --bootstrap ./tests/bootstrap.php --verbose ./tests",
         "translations": "cli/manipulate.translation.php -a format",
         "test": [
@@ -64,7 +65,8 @@
             "@phtml-lint",
             "@phpunit",
             "@phpcs",
-            "@phpstan"
+            "@phpstan",
+            "@phpstan-next"
         ],
         "fix": [
             "@translations",
diff --git a/tests/phpstan-next.txt b/tests/phpstan-next.txt
new file mode 100644
index 000000000..7f8d5e4e4
--- /dev/null
+++ b/tests/phpstan-next.txt
@@ -0,0 +1,93 @@
+# List of files, which are not yet passing PHPStan level 6 https://phpstan.org/user-guide/rule-levels
+# https://github.com/FreshRSS/FreshRSS/issues/4112
+# Used for automated tests to avoid regressions in files already passing that level.
+# Can be regenerated with something like:
+# find . -type d -name 'vendor' -prune -o -name '*.php' -exec sh -c 'vendor/bin/phpstan analyse --level 6 --memory-limit 512M {} >/dev/null 2>/dev/null || echo {}' \;
+
+./app/Controllers/extensionController.php
+./app/Controllers/feedController.php
+./app/Controllers/indexController.php
+./app/Controllers/updateController.php
+./app/Controllers/userController.php
+./app/Exceptions/AlreadySubscribedException.php
+./app/Exceptions/BadUrlException.php
+./app/Exceptions/FeedNotAddedException.php
+./app/Exceptions/ZipException.php
+./app/install.php
+./app/Mailers/UserMailer.php
+./app/Models/Auth.php
+./app/Models/BooleanSearch.php
+./app/Models/Category.php
+./app/Models/CategoryDAO.php
+./app/Models/CategoryDAOSQLite.php
+./app/Models/ConfigurationSetter.php
+./app/Models/Context.php
+./app/Models/DatabaseDAO.php
+./app/Models/DatabaseDAOPGSQL.php
+./app/Models/DatabaseDAOSQLite.php
+./app/Models/Entry.php
+./app/Models/Feed.php
+./app/Models/FeedDAO.php
+./app/Models/FeedDAOSQLite.php
+./app/Models/FilterAction.php
+./app/Models/FormAuth.php
+./app/Models/Log.php
+./app/Models/ReadingMode.php
+./app/Models/Search.php
+./app/Models/Share.php
+./app/Models/TagDAO.php
+./app/Models/TagDAOSQLite.php
+./app/Models/Themes.php
+./app/Models/UserDAO.php
+./app/Models/View.php
+./app/Services/ExportService.php
+./app/Services/ImportService.php
+./cli/_cli.php
+./cli/_update-or-create-user.php
+./cli/check.translation.php
+./cli/i18n/I18nCompletionValidator.php
+./cli/i18n/I18nData.php
+./cli/i18n/I18nFile.php
+./cli/i18n/I18nUsageValidator.php
+./cli/i18n/I18nValidatorInterface.php
+./cli/i18n/I18nValue.php
+./cli/manipulate.translation.php
+./lib/core-extensions/Google-Groups/extension.php
+./lib/core-extensions/Tumblr-GDPR/extension.php
+./lib/favicons.php
+./lib/http-conditional.php
+./lib/lib_date.php
+./lib/lib_install.php
+./lib/Minz/ActionController.php
+./lib/Minz/ActionException.php
+./lib/Minz/Configuration.php
+./lib/Minz/ConfigurationException.php
+./lib/Minz/ControllerNotActionControllerException.php
+./lib/Minz/ControllerNotExistException.php
+./lib/Minz/CurrentPagePaginationException.php
+./lib/Minz/Dispatcher.php
+./lib/Minz/Error.php
+./lib/Minz/Exception.php
+./lib/Minz/Extension.php
+./lib/Minz/ExtensionException.php
+./lib/Minz/ExtensionManager.php
+./lib/Minz/FileNotExistException.php
+./lib/Minz/FrontController.php
+./lib/Minz/Helper.php
+./lib/Minz/Log.php
+./lib/Minz/Mailer.php
+./lib/Minz/Migrator.php
+./lib/Minz/ModelArray.php
+./lib/Minz/ModelPdo.php
+./lib/Minz/Paginator.php
+./lib/Minz/Pdo.php
+./lib/Minz/PDOConnectionException.php
+./lib/Minz/PdoMysql.php
+./lib/Minz/PdoPgsql.php
+./lib/Minz/PdoSqlite.php
+./lib/Minz/PermissionDeniedException.php
+./lib/Minz/Request.php
+./lib/Minz/Session.php
+./lib/Minz/Translate.php
+./lib/Minz/Url.php
+./lib/Minz/View.php