array_udiff Computes the difference of arrays by using a callback function for data comparison. Description arrayarray_udiff arrayarray1 arrayarray2 array ... callbackdata_compare_func array_udiff returns an array containing all the values of array1 that are not present in any of the other arguments. Note that keys are preserved. For the comparison of the data data_compare_func is used. It must return an integer less than, equal to, or greater than zero if the first argument is considered to be respectively less than, equal to, or greater than the second. This is unlike array_diff which uses an internal function for comparing the data. <function>array_udiff</function> example priv_member = $val; } function comp_func_cr($a, $b) { if ($a->priv_member === $b->priv_member) return 0; return ($a->priv_member > $b->priv_member)? 1:-1; } } $a = array("0.1" => new cr(9), "0.5" => new cr(12), 0 => new cr(23), 1=> new cr(4), 2 => new cr(-15),); $b = array("0.2" => new cr(9), "0.5" => new cr(22), 0 => new cr(3), 1=> new cr(4), 2 => new cr(-15),); $result = array_udiff($a, $b, array("cr", "comp_func_cr")); print_r($result); ?> ]]> The result is: cr Object ( [priv_member:private] => 12 ) [0] => cr Object ( [priv_member:private] => 23 ) ) ]]> Two elements are considered equal if and only if (string) $elem1 === (string) $elem2. In words: when the string representation is the same. Please note that this function only checks one dimension of a n-dimensional array. Of course you can check deeper dimensions by using array_udiff($array1[0], $array2[0], "data_compare_func");. See also array_diff, array_diff_assoc, array_diff_uassoc, array_udiff_assoc, array_udiff_uassoc, array_intersect, array_intersect_assoc, array_uintersect, array_uintersect_assoc and array_uintersect_uassoc.