Rafael Dohms has, in a recent post to his blog, shown how to set up NetBeans to use PHPUnit tests with XDebug. This allows him to run his unit test and track down problems with the handy XDebug extension right from his IDE.
Every now and then you run into this weird situation in your code, where something that was supposed to zig is now zagging and it makes no sense whatsoever. For me this ends up happening in my unit tests since i'm not running everything in the browser everytime and since my tests usually run more scenarios then a regular browser run, that's where the weird stuff happens.
He shows you how to configure NetBeans with the path to your local PHP installation, configure the xdebug settings in your php.ini and how to coordinate the PHPUnit install to send its output to your NetBeans instance. Then it's as simple as clicking "Debug" on the project and running "phpunit-debug" from the command line.