qsort.php

<?php

function qsort(array $xs)
{
    $xs = array_values($xs);
    
    if (false === array_key_exists(1, $xs)) {
        return $xs;
    }
    
    $pivot = $xs[0];
    $xss = array_slice($xs, 1);
    $left = array_filter($xss, function ($x) use ($pivot) {
        return $x < $pivot;
    });
    $right = array_filter($xss, function ($x) use ($pivot) {
        return $x >= $pivot;
    });
    
    return array_merge(qsort($left), [$pivot], qsort($right));
}

$numbers = [2, 1, 4, 100, 3, 99];

var_dump(qsort($numbers));