Using usort() to Sort Arrays with a User-Defined Comparison Function in PHP

The PHP usort() function sorts the values of an array using a user‑defined comparison function, allowing custom ordering criteria.

Parameters: array &$array – the input array to be sorted; callable $cmp_function – a function that returns an integer less than, equal to, or greater than zero when the first argument is respectively less than, equal to, or greater than the second.

Return value: TRUE on success, FALSE on failure.

Example:

<?php
function cmp($a, $b) {
    if ($a == $b) {
        return 0;
    }
    return ($a < $b) ? -1 : 1;
}
$a = array(3, 2, 5, 6, 1);
usort($a, "cmp");
foreach ($a as $key => $value) {
    echo "$key: $value
";
}
?>

Output:

0: 1
1: 2
2: 3
3: 5
4: 6