Question

I’ve been fighting with this PHP-snippet for some hours and I can’t figure it out…
How does PHP determines the order when to pass the variables in this snippet:

function myfunction ($left, $right) {
    echo $left;
    echo '<br>';
    echo $right;

}

$a = array ('one', 'two', 'three', 'four');

usort($a, 'myfunction');

In this case, the variables are passed like this:

$left = 'two'   - $right = 'one';
$left = 'four'  - $right = 'two';
$left = 'three' - $right = 'two';
$left = 'four'  - $right = 'three';

But how is this sequence determined?!? I’ve been cracking my head and couldn’t find any logic…

Answer 1

usort uses the quicksort algorithm. You’ll see that if you change your function to return one of the expected values (-1, 0, 1), the results will be different:

function myfunction ($left, $right) {
    echo $left, ', ';
    echo $right, "<br>";
    if ($left == $right) {
        return 0;
    }
    return ($left  < $right) ? -1 : 1;

}

$a = array (1, 2, 3, 4);

usort($a, 'myfunction');

/* output:
2, 1
2, 3
4, 2
3, 2
4, 3
*/


/* output when null is returned, as your function does:
2, 1
4, 2
3, 2
4, 3
*/

From the article:

Quicksort first divides a large list into two smaller sub-lists: the
low elements and the high elements. Quicksort can then recursively
sort the sub-lists.

The steps are:

1. Pick an element, called a pivot, from the list.
2. Reorder the list so that all elements with values less than the pivot come before the pivot, while all elements with values greater
   than the pivot come after it (equal values can go either way). After
   this partitioning, the pivot is in its final position. This is called
   the partition operation.
3. Recursively sort the sub-list of lesser elements and the sub-list of greater elements.