/**
     * factorial($num) 计算阶乘
     * @param string $num
     * @return string $total
     */
    function factorial($num) {
        if (empty($num)) {
            return '输入不能为空！';
        }
        elseif ($num < 0){
            echo $num.'没有阶乘！';
        }
        elseif ($num == 0 || $num == 1) {
            echo $num.'的阶乘等于其本身！';
        }
        else {
            static $total = 1;
            $total *= $num;
            $num--;
            if ($num != 1) {
                factorial($num);
            }
            return $total;
        }
    }