array_diff_ukey Computes the difference of arrays using a callback function on the keys for comparison arrayarray_diff_ukey arrayarray1 arrayarray2 array ... callbackkey_compare_func array_diff_ukey 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. This comparison is done by a user supplied callback function. It must return an integer less than, equal to, or greater than zero if the first key is considered to be respectively less than, equal to, or greater than the second. $key2) return 1; else return -1; } $array1 = array('blue' => 1, 'red' => 2, 'green' => 3, 'purple' => 4); $array2 = array('green' => 5, 'blue' => 6, 'yellow' => 7, 'cyan' => 8); var_dump(array_diff_ukey($array1, $array2, 'key_compare_func')); ?> ]]> &example.outputs; int(2) ["purple"]=> int(4) } ]]> The two keys from the key => value pairs are considered equal only if (string) $key1 === (string) $key2 . In other words a strict type check is executed so the string representation must be the same. Please note that this function only checks one dimension of a n-dimensional array. Of course you can check deeper dimensions by using array_diff_ukey($array1[0], $array2[0], 'callback_func');. See also array_diff, array_udiff array_diff_assoc, array_diff_uassoc, array_udiff_assoc, array_udiff_uassoc, array_diff_key, array_intersect, array_intersect_assoc, array_intersect_uassoc, array_intersect_key and array_intersect_ukey.