下面我们以Thinkphp框架为示例来写一个小程序登录接口，并且通过解密方式获取unionid

/*
* 小程序登录API接口代码
*/
public function check() {
    $xcx = Config::get('config_api.xcx_user');

    $code = input('code');
    $encryptedData = input('encryptedData');
    $iv = input('iv');
    $referrer = input('user_id',0);

    $unionId = '';
    $unifo = [];

	$url = "https://api.weixin.qq.com/sns/jscode2session?appid=".$xcx['app_id']."&secret=".$xcx['app_secret']."&js_code=".$code."&grant_type=authorization_code";
	$content = file_get_contents($url); //如果这里返回不了数据，需要换别的方式

    $res = object_array(json_decode($content)); //返回openid，expires_in，session_key
    if(!isset($res)) return json(['code'=>1,'msg'=>'请求失败','res'=>$res]);

    if(empty($res['openid'])) return json(['code'=>1,'msg'=>'获取openid错误']);
    if(empty($res['session_key'])) return json(['code'=>1,'msg'=>'session_key获取失败']);
    
    if(empty($res['unionid'])){
        $pc = new WXBizDataCrypt($xcx['app_id'], $res['session_key']);
        $errCode = $pc->decryptData($encryptedData, $iv, $data );
        if ($errCode == 0) {
            $arr = object_array(json_decode($data));
            // 返回数据： Array ( [openId] => oGZUI0egBJY1zhBYw2KhdUfwVJJE [nickName] => Band [gender] => 1 [language] => zh_CN [city] => Guangzhou [province] => Guangdong [country] => CN [avatarUrl] => http://wx.qlogo.cn/mmopen/vi_32/aSKcBBPpibyKNicHNTMM0qJVh8Kjgiak2AHWr8MHM4WgMEm7GFhsf8OYrySdbvAMvTsw3mo8ibKicsnfN5pRjl1p8HQ/0 [unionId] => ocMvos6NjeKLIBqg5Mr9QjxrP1FA [watermark] => Array ( [timestamp] => 1477314187 [appid] => wx4f4bc4dec97d474b ) ) 
        } else {
            return json(['code'=>2,'msg'=>'获取unionId错误']);
        }
    }else{
        $unionId = $res['unionid'];
    }

    if(!empty($unionId)){
        $userInfo = UserModel::where( 'unionid',$unionId )->find();
    }else{
        return json(['code'=>1,'msg'=>'unionid获取失败']);
    }

    if(empty($userInfo)){ //生成用户
        $createData['unionid'] = $unionId;
        $createData['openid_xcx'] = $res['openid'];
        $createData['referrer'] = $referrer;
        $createData['last_login_time'] = time();
        $createData['last_login_ip'] = time();
        $createData['login_count'] = 1;
        $userObj = UserModel::createUser($createData);
    }else{
        //如果存在更新数据
        $updateData['last_login_time'] = time();
        $updateData['last_login_ip'] = time();
        $updateData['login_count'] = $userInfo['login_count']+1;
        $userObj = UserModel::where('id',$userInfo['id'])->update($updateData);
    }

    return json(['code'=>0,'msg'=>'登录成功','userInfo'=>$userInfo]);
}