Description:
------------
Adding elements to array resulting from array_diff is allocated with wrong numeric index key. 

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

$a1 = array(
    'one',
    'two',
    'three',
    'four'
);
$a2 = array_diff(
    $a1,
    array(
        'one',
        'two',
        'three',
        'four'
    )
);

$a3 = $a2;
$a3[] = 'five';

print_r($a3);

Expected result:
----------------
Array
(
    [0] => five
)

Actual result:
--------------
Output for PHP 4.3.0 - 5.4.32, 5.5.0 - 5.5.16, 5.6.0, php7@20140507 - 20140521, php7@20140801, hhvm-3.0.1 - 3.3.0 :

Array
(
    [0] => five
)

Output for PHP 5.4.34, 5.5.18, 5.6.1 - 5.6.2, php7@20140701, php7@20140901 - 20141001 :

Array
(
    [4] => five
)