/**
  * If oEmbed request wants XML, return XML instead of JSON.
  *
  * Basically a copy of {@link _oembed_rest_pre_serve_request()}. Unfortunate
  * that we have to duplicate this just for a URL check.
  *
  * @since 2.6.0
  *
  * @param bool                      $served  Whether the request has already been served.
  * @param WP_HTTP_ResponseInterface $result  Result to send to the client. Usually a WP_REST_Response.
  * @param WP_REST_Request           $request Request used to generate the response.
  * @param WP_REST_Server            $server  Server instance.
  * @return bool
  */
 public function oembed_xml_request($served, $result, $request, $server)
 {
     $params = $request->get_params();
     if (!isset($params['format']) || 'xml' !== $params['format']) {
         return $served;
     }
     // Validate URL against our oEmbed endpoint. If not valid, bail.
     // This is our mod to _oembed_rest_pre_serve_request().
     $query_params = $request->get_query_params();
     if (false === $this->validate_url_to_item_id($query_params['url'])) {
         return $served;
     }
     // Embed links inside the request.
     $data = $server->response_to_data($result, false);
     if (!class_exists('SimpleXMLElement')) {
         status_header(501);
         die(get_status_header_desc(501));
     }
     $result = _oembed_create_xml($data);
     // Bail if there's no XML.
     if (!$result) {
         status_header(501);
         return get_status_header_desc(501);
     }
     if (!headers_sent()) {
         $server->send_header('Content-Type', 'text/xml; charset=' . get_option('blog_charset'));
     }
     echo $result;
     return true;
 }