找不到PHP错误类'Google_Http_Request'

I'm trying to download or convert a file from Google Drive. I am using the below code, and I get the error "Class 'Google_Http_Request' not found". I can't seem to find what I'm missing. Am I missing something? A required file?

I'm using php-google-api-php-client (0.6.2-1) [universe].

Thanks.

$file = $service->files->get($drive_id);
        //if actual file
        //$downloadUrl = $file->getDownloadUrl();
        //if a google doc
        $downloadUrl = $file->getExportLinks();


        if ($downloadUrl) {
            $request = new Google_Http_Request($downloadUrl, 'GET', null, null);
            $httpRequest = $service->getClient()->getAuth()->authenticatedRequest($request);
            if ($httpRequest->getResponseHttpCode() == 200) {
              return $httpRequest->getResponseBody();
            } else {
              // An error occurred.
              return null;
            }
        } else {
            // The file doesn't have any content stored on Drive.
            return null;
        }

In general, php does not autoload any classes. In basic terms, you would need to require() the right file to load Google_Http_Request, but you will run into more issues as google drive api requires authentication.

I suggest you follow much newer instructions provided on the Google's Developer Center: https://developers.google.com/drive/web/quickstart/php. The quickstart even goes as far as list files.