zendframework/zf2 — pull request #2135 by Thinkscape (2012-08-08 18:18:34+00:00)
Title: Change all 'callback' typehints to 'callable' 

Make the code PHP 5.4 friendly.

`callback` was a pseudo-type used in old documentation.

`callable` (along with `is_callable()`) is a the new reference to things that can be called. Starting with PHP 5.4 it is also an official type hint.

Reference:
- http://php.net/manual/en/language.types.callable.php
- http://php.net/manual/en/function.is-callable.php
- http://nl.php.net/manual/en/language.pseudo-types.php#language.types.callback