clean_comment_cache

			 *
			 * @since 2.9.0
			 *
			 * @param string $concat_useragent    The user agent concatenated with ' -- WordPress/'
			 *                                    and the WordPress version.
			 * @param string $useragent           The useragent.
			 * @param string $pingback_server_url The server URL being linked to.
			 * @param string $pagelinkedto        URL of page linked to.
			 * @param string $pagelinkedfrom      URL of page linked from.
			 */
			$client->useragent = apply_filters( 'pingback_useragent', $client->useragent . ' -- WordPress/' . get_bloginfo( 'version' ), $client->useragent, $pingback_server_url, $pagelinkedto, $pagelinkedfrom );
			// When set to true, this outputs debug messages by itself.
			$client->debug = false;

			if ( $client->query( 'pingback.ping', $pagelinkedfrom, $pagelinkedto ) || ( isset( $client->error->code ) && 48 == $client->error->code ) ) { // Already registered.
				add_ping( $post, $pagelinkedto );