Debian testing updated to PHP 8.1 recently, so now my development wiki is throwing deprecation warnings like this:
Deprecated: MapCacheLRU implements the Serializable interface, which is deprecated. Implement __serialize() and __unserialize() instead (or in addition, if support for old PHP versions is necessary) in /home/taavi/src/mediawiki/includes/libs/MapCacheLRU.php on line 36
and
Deprecated: RawMessage implements the Serializable interface, which is deprecated. Implement __serialize() and __unserialize() instead (or in addition, if support for old PHP versions is necessary) in /var/www/wiki/mediawiki/core/includes/RawMessage.php on line 35
See also: https://php.watch/versions/8.1/serializable-deprecated