Description:
------------
Hi,

recently realized that isset not returning the same result as array_key_exists 
does for int-Keys.

Test script:
---------------
<?php
$foo = array(null);

var_dump( isset($foo[0]), array_key_exists(0, $foo) );

Expected result:
----------------
bool(true)
bool(true)

Actual result:
--------------
bool(false)
bool(true)