A couple months ago I learned of Xdebug, started using it, and now I can't imagine living without a debugger. Somewhere on SO, I found out about another debugger. Now I'm not sure if I should continue using Xdebug or not. I decided to search the Web for a comparison of PHP debuggers but came back empty-handed.
What PHP debugger do you use and why?
http://www.php-debugger.com/dbg/
It comes with precompiled binaries for win32 and linux and is quite easy to install and use.
Xdebug works good for me. Also if you use an IDE like Eclipse or Netbeans, it has a debugger built it and you can step through code set breakpoints, etc.
Zend Studio has a built-in debugger, but it's a total hack and doesn't work against any frameworks that use output buffering.
I ended up using XDebug, too - it integrates very well with Komodo, and supports remote debugging.