In a recent post to his blog Mark Hamlin talks about some of his difficulties in getting XDebug and Netbeans working together from an OSX machine hitting a Ubuntu server.

For the past 18 months working with PHP, i've primarily used alternatives, not out of preference, but because netbeans xdebug integration consistently failed me. It would (might) work with a remote apache, but would not play with scripts executed remotely from the command line. I could be fairly sure my xdebug config was sound as I no problems with MacGDB and PHPStorm whatsoever.

With a little more investigation, he discovered that it was the OSX firewall causing issues. He found that, with a new incoming connection came a confirmation box to approve the connection. This, of course, wasn't passed along to Netbeans so he never saw it. In the end, he set up a reverse SSH tunnel to bypass the firewall completely (command included).