<?php
function key_compare_func($a,$b){
    if($a === $b){
        return 0;
    }
    return($a>$b)?1:-1;
}
$a = array('a'=>'green','b'=>'brown','c'=>'blue','pink','red');
$b = array('a'=>'green','b'=>'yellow','red');
print_r(array_diff_ukey($a,$b,"key_compare_func"));
Array
(
    [c] => blue
    [1] => red
)