Test a WebSocket using php fsockopen, bit chaotic, but working for me. You can use that for a HTTP Sensor in PRTG or Nagios

websocket-check.php

<?php

error_reporting(-1);

$host = "your-host.tld"; // your websocket url without protocol part
$SecWebsocketKey = "your key from websocket connection"; // try wireshark or developer Tools to get this

$origin = "http://$host/"; // origin for the header

$host = 'localhost';  //where is the websocket server
$port = 8080; // Your Websocket Port



$head = "GET / HTTP/1.1"."\r\n".
	"Upgrade: WebSocket"."\r\n".
	"Connection: Upgrade"."\r\n".
	"Origin: $$origin"."\r\n".
	"Host: $host"."\r\n".
	"Content-Length: 0\r\n".
	"Sec-WebSocket-Key: $SecWebsocketKey"."\r\n".
	"Sec-WebSocket-Protocol: wamp"."\r\n".
	"Accept-Encoding: gzip, deflate"."\r\n".
	"Accept-Language: de-DE,de;q=0.9,en-US;q=0.8,en;q=0.7"."\r\n".
	"Sec-WebSocket-Extensions: permessage-deflate; client_max_window_bits"."\r\n".
	"Sec-WebSocket-Version: 13"."\r\n"
	."\r\n";
			
// WebSocket Handshake
$sock = fsockopen($host, $port, $errno, $errstr, 2);
fwrite($sock, $head ) or die('error:'.$errno.':'.$errstr);
$headers = fread($sock, 2000);
fclose($sock);

// Check for Protocol Upgrade Header in the Response
if(strpos($headers, 'HTTP/1.1 101 Switching Protocols') !== false) {
	echo "OK";
} else {
	echo "Websocket Connection Error";
}