Unassigned at IANA, but the port every Swoole PHP example listens on — so a PHP coroutine app server is the usual answer.
IANA has no assignment for 9501. In practice it is the number baked into Swoole's own documentation and README examples: new Swoole\Http\Server('127.0.0.1', 9501) and the matching WebSocket server. Swoole is a PHP extension that turns PHP into a long-running event-driven server, so a listener here is usually a Laravel Octane, Hyperf, or hand-rolled Swoole process that a copy-pasted example never renamed. Anything else on 9501 is an application picking a free high port.
Expose with care
An app server, not a proxy — it usually has no TLS, no request-size limits, and no access logging of its own. Put it behind nginx or a load balancer and bind it to localhost.
$ curl -sI http://TARGET:9501/Seen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.