see the function array_diff_ukey in the official manual. The examples in

<?php
function key_compare_func($key1, $key2)
{
    if ($key1 == $key2)
        return 0;
    else if ($key1 > $key2)
        return 1;
    else
        return -1;
}

$array1 = array("blue"  => 1, "red"  => 2, "green"  => 3, "purple" => 4);
$array2 = array("green" => 5, "blue" => 6, "yellow" => 7, "cyan"   => 8);

var_dump(array_diff_ukey($array1, $array2, "key_compare_func"));

key_compare_func directly compares the size of strings in key?