php 生成6位隨機字符串

   /**
     * 生成隨機字母加數字 字符串
     * @param int $len 要生成字符串長度
     * @param null $chars 要生成字符串包含的字符
     * @return string   生成的字符串
     */
    public function getRandomString($len = 6, $chars = null)
    {
        if (is_null($chars)) {
            $chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
        }
        mt_srand(10000000 * (double)microtime());
        for ($i = 0, $str = '', $lc = strlen($chars) - 1; $i < $len; $i++) {
            $str .= $chars[mt_rand(0, $lc)];
        }
        return $str;
    }