https://codesearch.wmflabs.org/search/?q=5%5C.3&i=nope&files=%5C.php&repos= lists some back-compatible code that was written for the PHP 5.3 era.
This is no longer needed because [all MediaWiki versions that are nowadays supported require at least PHP version 5.5.9](https://en.wikipedia.org/wiki/MediaWiki_version_history).
Go through that output, find two specific instances of such code, and clean them up in a patch!
- [[ https://codesearch.wmflabs.org/search/?q=%5C%24(self%7Cthat)%20%3D%20%5C%24this%3B&i=nope&files=&repos= | Implicit binding of `$this` in closures ]]
- [[ https://codesearch.wmflabs.org/search/?q=(PHP%7C%20)(5%5C.%5B1-6Xx%5D%7C7%5C.%5B02%5D)%7CPHP%20%3F%5B57%5D%5B%5E.%5D&i=fosho&files=%5C.php%24&repos= | Mentions of old PHP versions in code ]]