On PHPFreaks.com there's a new tutorial providing a beginner's guide to debugging in PHP (with the built in functionality PHP has, not external software).

Everyday the forums see probably hundreds of topics posted where the problem is a fairly simple error. [...] As a beginner, it can be difficult to find and solve these errors. By tackling each of these in turn, I hope to teach you some methods of finding and solving them.

They look at the different sorts of errors - syntax errors, fatal errors, warnings, notices - as well as some of the ones a bit harder to track down like database problems and logical errors.