parseStr($template); $template = $DOM->query('body'); // url is a temporary data tag that will be further replaced with // the proper track API URL during sending $url = Links::DATA_TAG_OPEN; $openTrackingImage = sprintf( '', $url ); $template->html($template->html() . $openTrackingImage); return $DOM->__toString(); } public static function addTrackingImage() { WPFunctions::get()->addFilter(Renderer::FILTER_POST_PROCESS, function ($template) { return OpenTracking::process($template); }); return true; } }