in cluster.php, around line 70, after
$result = $rcon->send('ast ping');

the script looks for files created by the 'pong' case block:
if (file_exists($filename)) {
...

These files do not get created until we get an 'pong' command from the server. Adding sleep() as you've done inside the foreach ping loop does not affect the result. The only way I was able to get a green result was to paste the ping-*.test files in while the foreach ping loop was running.

Is it may be because the "case 'pong'" is happening outside the foreach ping block? It would have to exit the ping block to catch the pong command that writes the files - by then the script has already decided the files don't exist.

Would it not be sufficient to do:
if ($result == 'ping done.') {
echo '<strong style="color: green;">good</strong>.';
...
?

Do we really care that the pong command created the files if the rcon response is good?

I'm running php-8.1.12 with fast-cgi from nginx-1.23.2 on Windows 11.