クイックソート

// quick sort
    function quick_sort(&$arr){
        if (count($arr) < 2) return $arr;
        $left = $right = array();
        $pivot = array_shift($arr);
        foreach ($arr as $v) {
            if ($v < $pivot){
                $left[] = $v;
            } else {
                right[] = $v;
            }
        }
        quick_sort($left);
        quick_sort($right);
        $arr = array_merge($left, array($pivot), $right);
    }


    $arr = array(1,100,24,40,12,4);
    quick_sort($arr);
    echo implode(', ', $arr)."\n";