public function getAccessToken()
    {
        // get token from cache file
        $result = $this->readAccessToken();
        if ('' != $result) {
            return $result;
        }

        // get token from we chat api
        $data   = $this->getAccessTokenFromWeChat();
        // save token to cache data
        $this->saveAccessToken($data['token'], $data['expire']);

        return $data['token'];
    }