(PHP 4 >= 4.0.1, PHP 5)
array_diff — Computes the difference of arrays
Compares array1 against array2 and returns the difference.
The array to compare from
An array to compare against
More arrays to compare against
Returns an array containing all the entries from array1 that are not present in any of the other arrays.
Example #1 array_diff() example
$array1 = array("a" => "green", "red", "blue", "red");
$array2 = array("b" => "green", "yellow", "red");
$result = array_diff($array1, $array2);
Multiple occurrences in $array1 are all treated the same way. This will output :
Array (  => blue )
Note: Two elements are considered equal if and only if (string) $elem1 === (string) $elem2. In words: when the string representation is the same.
Note: This function only checks one dimension of a n-dimensional array. Of course you can check deeper dimensions by using array_diff($array1, $array2);.
This was broken in PHP 4.0.4!