Description:
------------
Using an anonymous function as argument #2 in preg_replace_callback issues the following warning:
preg_replace_callback() [function.preg-replace-callback]: Requires argument 2, '', to be a valid callback

This warning was not thrown previously using PHP 5.3.1

Test script:
---------------
echo preg_replace_callback('~-([a-z])~', function ($match) {
    return strtoupper($match[1]);
}, 'hello-world');

Expected result:
----------------
helloworld

Actual result:
--------------
hello-world + warning