Cheers,
I ended up getting a debugging regime working, I'll post it here for reference:
1) Have a local server. For windows, I have EasyPHP DevServer.
2) Make sure it has XDebug installed. I updated my XDebug on my local server to the latest version as follows:
a) On your local server, make a php file which calls phpinfo(), then open that file in browser.
b) copy and paste that output to
http://xdebug.org/wizard.php
c) follow that wizard's instructions
3) Get the latest Netbeans IDE, and install the plugin called "PHP".
4) In Tools > Options > PHP > Debugging, unselect 'Stop at First Line'. If you don't, especially when you're trying to debug something as big as MyBB, it will trigger an invisible breakpoint at the first line of every single php file.
5) Create a New Project in NetBeans, Category:PHP > PHP application with Existing Source
6) Place breakpoints as needed.
7) Click Debug > Debug Project (or CTRL+F5)
8) Profit