How to understand usort logic?

Question

I'm trying to understand how php function usort works. I have such code:

<?php 
$users[] = array('login' => 'moon', 'name' => 'Chris');
$users[] = array('login' => 'star', 'name' => 'Piter');
$users[] = array('login' => 'mars', 'name' => 'Tim');
$users[] = array('login' => 'earth', 'name' => 'Garry');

function compare($a, $b) {                  
    echo $a['login'] . '--' . $b['login'] . '<br />';
    echo strcmp($a['login'], $b['login']) . '<br />';
    return strcmp($a['login'], $b['login']);              
}
usort($users, "compare");

echo '<pre>'; print_r($users); echo '</pre>';
?>

It will output such result:

star--moon
1
star--mars
1
earth--star
-1
moon--earth
1
mars--moon
-1
earth--mars
-1
Array
(
    [0] => Array
        (
            [login] => earth
            [name] => Garry
        )

    [1] => Array
        (
            [login] => mars
            [name] => Tim
        )

    [2] => Array
        (
            [login] => moon
            [name] => Chris
        )

    [3] => Array
        (
            [login] => star
            [name] => Piter
        )

)

As far as I understand second param should be comparison function and it can only return 3 values (-1,0,1) and usort use this results to sort array? Also I read, usort use Quicksort implementation to sort array. That is why star is first and moon - second? Quicksort divide array into two parts and then sort it? And can I implement this function for 2,3 dimensions array?

Answer 1

You're telling usort which item to place first when it compares any two items in your array. Your function is returning the strcmp comparison of the login values of the elements, so it's putting all of your elements in alphabetical order based on the login name.

Answer 2

Yes, usort uses the comparison function, to compare the values and sort the array with the quicksort algorithm. From http://php.net/manual/en/function.usort.php:

The comparison function must return an integer less than, equal to, or greater than zero if the first argument is considered to be respectively less than, equal to, or greater than the second.

Links to the implementation of usort in PHP can be found here: What sort algorithm does PHP use?. According to http://murilo.wordpress.com/2011/02/05/phps-sort-functions-are-bad-designed/ the algorithm uses the middle element as the pivot element, implemented as this:

offset = (end - begin) >> 1;

This should be why the algorithm uses 'star' as the first pivot element.

For multidimensional arrays, use uasort if you want to maintain index association. This question contains an example of sorting multidimensional arrays.