- var_dump: documentation ( source)
- array_merge: documentation ( source)
- array_values: documentation ( source)
- array_diff: documentation ( source)
<?php
$a = array_key( array('photo' => 'foo.jpeg', 'audio' => 'bar.mp3') );
$b = array('photo', 'audio', 'video');
var_dump(array_values(array_merge(array_diff($b, $a), array_diff($a, $b))));