Quick PHP Quiz

<?php
function list_cmp($a, $b) {
    global $order;

    foreach ($order as $key => $value) {
        if ($a == $value) {
            return 0;
        }

        if ($b == $value) {
            return 1;
        }
    }
}

$order[0] = 1;
$order[1] = 3;
$order[2] = 4;
$order[3] = 2;

$array[0] = 2;
$array[1] = 1;
$array[2] = 3;
$array[3] = 4;
$array[4] = 2;
$array[5] = 1;
$array[6] = 2;

usort($array, "list_cmp");
?>

/*
What is the logic / thinking / explanation of:
1) Comparing a value of $a to a value of the $order array and if they are the same doing nothing (return 0)
2) If $a is not equal to a value of the $order array, comparing $b to a value of the $order array, and if $b equals a value of the $order array, returning 1, which moves the (current) $b variable towards the end of the array (higher index)?
The goal is to sort the $array based on the order of the $order array, but what is the explanation of achieving this with how this code is set up? I am not getting this yet…
*/