code: public function post($request)
  	{
  		if ($request instanceof Request) {
  			$request = $request->xml();
  		}
  
  		$ch = curl_init();
  		curl_setopt($ch, CURLOPT_URL, $this->url);
  		if ($this->user_agent != null) {
  			curl_setopt($ch, CURLOPT_USERAGENT, $this->user_agent);
  		}
  		curl_setopt($ch, CURLOPT_HEADER, 0); // no headers in the output
  		curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1); // return instead of output
  		curl_setopt($ch, CURLOPT_POST, 1);
  		curl_setopt($ch, CURLOPT_POSTFIELDS, $request);
  		curl_setopt($ch, CURLOPT_HTTPHEADER, array(
  			'Content-type: application/xml; charset=utf-8',
  			));
  		$response = curl_exec($ch);
  		curl_close($ch);
  
  		if (empty($response)) {
  			return null;
  		}
  
  		return $response;
  	}
docs: Post xml document to the NCIP service
  
  @param  Request  $request
  @return string