MediaWiki doesn't work if the mbstring.func_overload PHP is non-zero*.
Should not then the MediaWiki code set this value explicitly? I was thinking likely in the Language.php file where it sets the internal encoding.
Or at very least, should not an assertion of sorts be done to confirm that this is 0?
*See my problem http://wiki.disemia.com/Help_-_Corruption_of_Non-ASCII_Characters