PHP Manual . ... PHP 4 ) array_diff -- Computes the difference of arrays . array array_diff ( array array1, array array2 [, array ...]) . array_diff() returns an array containing all the values of array1 that are not present in any of the other arguments. ... Example 1. array_diff() example . array1 = array ("a" => "green", "red", "blue", "red"); $array2 = array ("b" => "green", "yellow", "red"); $result = array_diff ($array1, $array2); . This makes $result have array ("blue"); . ...