Carsten Lucke lists out a a combination of tools he's found that can make debugging your PHP applications a simple task - both for free.
I figured out that the PHPEclipse people were developing a plugin to use XDebug but there didn't seem to be much progress. So I stopped my efforts on free remote-debugging. Some weeks ago XDebug2 was released and I decided to give the remote-debugging thing one more shot. I tried PDT + XDebug2 and guess what - it worked out fine!
He includes a brief overview of what you'll need and how to get it installed, but you'll still need to be a bit familiar with how Eclipse and the XDebug software work to get things running smoothly.