thinkphp微信开之安全模式消息加密解密不成功的解决办法

/**
   * 对密文进行解密
   * @param string $encrypt 密文
   * @return string     明文
   */
  public function decrypt($encrypt){
    //BASE64解码
    $encrypt = base64_decode($encrypt);
    //打开加密算法模块
    $td = mcrypt_module_open(MCRYPT_RIJNDAEL_128, '', MCRYPT_MODE_CBC, '');
    //初始化加密算法模块
    mcrypt_generic_init($td, $this->cyptKey, substr($this->cyptKey, 0, 16));
    //执行解密
    $decrypt = mdecrypt_generic($td, $encrypt);
    //去除PKCS7补位
    $decrypt = self::PKCS7Decode($decrypt, mcrypt_enc_get_key_size($td));
    //关闭加密算法模块
    mcrypt_generic_deinit($td);
    mcrypt_module_close($td);
    if(strlen($decrypt) < 16){
      throw new \Exception("非法密文字符串！");
    }
    //去除随机字符串
    $decrypt = substr($decrypt, 16);
    //获取网络字节序
    $size = unpack("N", substr($decrypt, 0, 4));
    $size = $size[1];
    //APP_ID
    $appid = substr($decrypt, $size + 4);
    //验证APP_ID
    if($appid !== $this->appId){
      throw new \Exception("非法APP_ID！");
    }
    //明文内容
    $text = substr($decrypt, 4, $size);
    return $text;
  }
  /**
   * PKCS7填充字符
   * @param string $text 被填充字符
   * @param integer $size Block长度
   */
  private static function PKCS7Encode($text, $size){
    //字符串长度
    $str_size = strlen($text);
    //填充长度
    $pad_size = $size - ($str_size % $size);
    $pad_size = $pad_size ? : $size;
    //填充的字符
    $pad_chr = chr($pad_size);
    //执行填充
    $text = str_pad($text, $str_size + $pad_size, $pad_chr, STR_PAD_RIGHT);
    return $text;
  }
  /**
   * 删除PKCS7填充的字符
   * @param string $text 已填充的字符
   * @param integer $size Block长度
   */
  private static function PKCS7Decode($text, $size){
    //获取补位字符
    $pad_str = ord(substr($text, -1));
    if ($pad_str < 1 || $pad_str > $size) {
      $pad_str= 0;
    } 
      return substr($text, 0, strlen($text) - $pad_str);
  }

 <xml>
 <ToUserName><![CDATA[gh_aebd]]><\/ToUserName>\n
 <FromUserName><![CDATA[oopVmxHZaeQkDPsRcbpwXKkH-JQ]]><\/FromUserName>\n
 <CreateTime><\/CreateTime>\n
 <MsgType><![CDATA[text]]><\/MsgType>\n
 <Content><![CDATA[\uecf\u]]><\/Content>\n
 <MsgId><\/MsgId>\n
 <\/xml>

 //明文内容
     $text = substr($decrypt, , $size);
 //去掉多余的内容
     $text=str_replace('<\/','</', $text);   
     $text=str_replace('>\n','>', $text);
     return $text;