Description:
------------
array_filter($some_array, 'isset') cause PHP WARNING.

Test script:
---------------
<?php

$foo = array('a' => 1, 'b' => null);

var_export(array_filter($foo, 'isset');

Expected result:
----------------
array (
  'a' => 1,
)

Actual result:
--------------
PHP Warning:  array_filter() expects parameter 2 to be a valid callback, function 'isset' not found or invalid function name in php shell code on line 1
PHP Stack trace:
PHP   1. {main}() php shell code:0
PHP   2. array_filter() php shell code:1
NULL