To anybody wanting a double-sided array_diff - mentioned by rudigier at noxx dot at. Remember, array_diff gives you everything in the first array that isn't in the subsequent arrays.
$array1=array('blue','red','green');
$array2=array('blue','yellow','green');
array_merge(array_diff($array1, $array2),array_diff($array2, $array1));
Result
------
Array
(
[0] => red
[1] => yellow
)