array_diff_key Computes the difference of arrays using keys for comparison arrayarray_diff_key arrayarray1 arrayarray2 array ... array_diff_key returns an array containing all the values of array1 that have keys that are not present in any of the other arguments. Note that the associativity is preserved. This function is like array_diff except the comparison is done on the keys instead of the values. 1, 'red' => 2, 'green' => 3, 'purple' => 4); $array2 = array('green' => 5, 'blue' => 6, 'yellow' => 7, 'cyan' => 8); var_dump(array_diff_key($array1, $array2)); ?> ]]> The printout of the above program will be: int(2) ["purple"]=> int(4) } ]]> The two keys from the key => value pairs are considered equal only if (string) $key1 === (string) $key2 . In other words a strict type check is executed so the string representation must be the same. Please note that this function only checks one dimension of a n-dimensional array. Of course you can check deeper dimensions by using array_diff_key($array1[0], $array2[0]);. See also array_diff, array_udiff array_diff_assoc, array_diff_uassoc, array_udiff_assoc, array_udiff_uassoc, array_diff_ukey, array_intersect, array_intersect_assoc, array_intersect_uassoc, array_intersect_key and array_intersect_ukey.