Built in support for sets in PHP?

I'm looking for a simple way to create an array in php that will not allow duplicate entries, but allows for easy combining of other sets or arrays.

I'm mostly interested in whether such a feature exists in the language because writing my own wouldn't be difficult. I just don't want to if I don't need to.


Just an idea, if you use the array keys instead of values, you'll be sure there are no duplicates, also this allows for easy merging of two "sets".

$set1 = array ('a' => 1, 'b' => 1, );
$set2 = array ('b' => 1, 'c' => 1, );
$union = $set1 + $set2;