Check and return duplicates array php

Question

I would like to check if my array has any duplicates and return the duplicated values in an array. I want this to be as efficient as possible.

Example:

Answer 1

this will be ~100 times faster than array_diff

$dups = array();
foreach(array_count_values($arr) as $val => $c)
    if($c > 1) $dups[] = $val;