php infinite classification (recursive)

public function Tree($arr, $pid = 0, $level = 0)
    {
        static $list = [];
        foreach ($arr as $key => $value) {
            if ($value["pid"] == $pid) {
                $value["level"] = $level;
                $list[] = $value;
                unset($arr[$key]); //删除已经排好的数据为了减少遍历的次数，当然递归本身就很费神就是了
                $this->Tree($arr, $value["id"], $level + 1);
            }
        }
        return $list;

    }