Xdebug 3's default debug port — the IDE listens, PHP connects back to it.
Xdebug 3 changed its default from 9000 (which collided with PHP-FPM) to 9003 for xdebug.client_port, and the DBGp command-line client uses the same number. The direction surprises people: the debugged PHP process is the client, and your editor or IDE is the thing listening on 9003. So a listener here is almost always a developer's IDE, and IANA has 9003–9004 marked Unassigned.
Do not expose
A DBGp listener accepts a debug session from anyone who can reach it and lets that session evaluate code in the debugger's context; bind it to loopback and reach remote hosts over an SSH tunnel.
$ sudo lsof -nP -iTCP:9003 -sTCP:LISTENSeen 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.