array_diff Computes the difference of arrays arrayarray_diff arrayarray1 arrayarray2 array ... array_diff returns an array containing all the values of array1 that are not present in any of the other arguments. Note that keys are preserved. "green", "red", "blue", "red"); $array2 = array ("b" => "green", "yellow", "red"); $result = array_diff ($array1, $array2); ]]> This makes $result have array ("blue");. Multiple occurrences in $array1 are all treated the same way. Two elements are considered equal if and only if (string) $elem1 === (string) $elem2. In words: when the string representation is the same. This was broken in PHP 4.0.4! See also array_intersect.