If you're not getting a count(array_diff($a1,$a2))>0 with something similar to the following arrays should use the php.net/array_diff_assoc function instead.

$a1 = Array
(
    [0] => id
    [1] => id_1
    [2] => id_2
)

$a2 = Array
(
    [0] => id
    [1] => id_2
    [2] => id_1
)