Logging debugging communication:
xdebug.log=/tmp/remote.log
Also shows connection attempts and failures:
[3553807] Log opened at 2021-05-14 09:46:31.329802
[3553807] [Step Debug] INFO: Connecting to configured address/port: localhost:9003.
[3553807] [Step Debug] WARN: Creating socket for 'localhost:9003', poll success,
but error: Operation now in progress (29).
[3553807] [Step Debug] ERR: Could not connect to debugging client.
Tried: localhost:9003 (through xdebug.client_host/xdebug.client_port) :-(