public function GetOAuthHeader($url) {
        $signatureMethod = new OAuthSignatureMethod_HMAC_SHA1();
        $request = OAuthRequest::from_consumer_and_token($this->consumer,
                $this->token, 'POST', $url, array());
        $request->sign_request($signatureMethod, $this->consumer, $this->token);
        $header = $request->to_header();
// Remove "Authorization:" prefix.
        $pieces = explode(':', $header, 2);
        return trim($pieces[1]);
    }