function prepare_authentication_header($url)
 {
     $signature_method = new Google_Analytics_OAuthSignatureMethod_HMAC_SHA1();
     $consumer = new Google_Analytics_OAuthConsumer('anonymous', 'anonymous', NULL);
     $token = new Google_Analytics_OAuthConsumer($this->oauth_token, $this->oauth_secret);
     $oauth_req = Google_Analytics_OAuthRequest::from_consumer_and_token($consumer, $token, 'GET', $url, array());
     $oauth_req->sign_request($signature_method, $consumer, $token);
     $headers = $oauth_req->to_header();
     $headers = explode(": ", $headers);
     $headers[$headers[0]] = $headers[1];
     return $headers;
 }