apply_filters( 'pingback_useragent', string $concat_useragent, string $useragent, string $pingback_server_url, string $pagelinkedto, string $pagelinkedfrom )

3161			 *                                    and the WordPress version.3162			 * @param string $useragent           The useragent.3163			 * @param string $pingback_server_url The server URL being linked to.3164			 * @param string $pagelinkedto        URL of page linked to.3165			 * @param string $pagelinkedfrom      URL of page linked from.3166			 */3167			$client->useragent = apply_filters( 'pingback_useragent', $client->useragent . ' -- WordPress/' . get_bloginfo( 'version' ), $client->useragent, $pingback_server_url, $pagelinkedto, $pagelinkedfrom );3168			// When set to true, this outputs debug messages by itself.3169			$client->debug = false;3170 3171			if ( $client->query( 'pingback.ping', $pagelinkedfrom, $pagelinkedto ) || ( isset( $client->error->code ) && 48 == $client->error->code ) ) { // Already registered.3172				add_ping( $post, $pagelinkedto );3173			}