This was detected as part of MDL-73587 (PHPCompatibility complete run against codebase).

INI directive 'mbstring.func_overload' is deprecated since PHP 7.2 and removed since PHP 8.0

And we have a couple of cases in the mod_wiki activity:

• mod/wiki/classes/external.php : 516
• mod/wiki/tests/externallib_test.php : 655

Note this is not critical because it's returning the same value than previously, but better get rid of them once and forever.

Ciao