谷歌驱动器api无法访问私有属性google_Client::$io

下午好！我正在尝试下载该文件。我的脚本：

function GetFile($service, $fileId) {
    $fileVars = null;
    try {
        $file = $service->files->get($fileId);
        $fileVars = get_object_vars($file);
        $downloadUrl = $file->getDownloadUrl();
        error_log('Download URL file from Drive: ' . $downloadUrl);
        if ($downloadUrl) {
            $request = new Google_Http_Request($downloadUrl, 'GET', null, null); 
            $httpRequest = Google_Client::$io->authenticatedRequest($request);
            error_log(print_r($httpRequest, 1));
            if ($httpRequest->getResponseHttpCode() == 200) {
                $content = $httpRequest->getResponseBody();
                $fileVars['content'] = $content?($content):'';
            } else {
                return null;
            }
        } else {
            return null;
        }
    } catch (apiServiceException $e) {
        error_log('Error retrieving file from Drive: ' . $e->getMessage());
        throw $e;
    }
    return json_encode($fileVars);
}
$requestedFile = GetFile($service,$data[0]['id']);

我推断致命错误：无法访问私有属性Google_Client:：$io

告诉我，我犯了什么错？提前感谢，抱歉英语不好！