[ticket/13168] Fix mbstring warnings in ACP for PHP 5.6 compatibility

PHPBB3-13168
Merge pull request #3047 from Elsensee/ticket/13168

[ticket/13168] Fix mbstring warnings in ACP for PHP 5.6 compatibility

* Elsensee/ticket/13168:
  [ticket/13168] Fix mbstring warnings in ACP for PHP 5.6 compatibility