How the usort() sorting algorithm works?

I have an usort() example and I added some echo statements to see how the code works:

<?php
function list_cmp($a, $b) {
    global $order;
    echo "\$a=$a, \$b=$b </br>";

    foreach ($order as $key => $value) {
        echo "\$value=$value </br>";
        if ($a == $value) {
            echo "\$a=\$value, returing 0. </br>";
            return 0;
        }
        if ($b == $value) {
            echo "\$b=\$value, returing 1. </br>";
            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");
?>

The output of the code is this:

$a=2, $b=1 
$value=1 
$b=$value, returing 1. 
$a=2, $b=3 
$value=1 
$value=3 
$b=$value, returing 1. 
$a=1, $b=3 
$value=1 
$a=$value, returing 0. 
$a=2, $b=4 
$value=1 
$value=3 
$value=4 
$b=$value, returing 1. 
$a=3, $b=4 
$value=1 
$value=3 
$a=$value, returing 0. 
$a=2, $b=2 
$value=1 
$value=3 
$value=4 
$value=2 
$a=$value, returing 0. 
$a=2, $b=1 
$value=1 
$b=$value, returing 1. 
$a=2, $b=1 
$value=1 
$b=$value, returing 1. 
$a=4, $b=1 
$value=1 
$b=$value, returing 1. 
$a=3, $b=1 
$value=1 
$b=$value, returing 1. 
$a=1, $b=1 
$value=1 
$a=$value, returing 0. 
$a=2, $b=2 
$value=1 
$value=3 
$value=4 
$value=2 
$a=$value, returing 0. 

What is the mechanism of creating the 12 $a-$b pairs - 2-1, 2-3, 1-3, 2-4, 3-4, 2-2, 2-1, 2-1 (the same again?), 4-1, 3-1, 1-1, 2-2. The above code returns 1,1,0,1,0,0,1,1,1,1,0,0. And also what is the mechanism of sorting the array, based on the values that get returned? I am trying to understand how the usort() mechanism works.

Thanks.