In these days, working with PHP after a lot of time, I’ve installed this cool tool for debugging PHP (it supports both PHP 4 and 5).
XDebug is really useful for debugging PHP, which is usually not really easy to debug, and also easy to install.
It provides a lot of valuable debug informations, for example a full traceback for errors, profilig with memory and CPU usage, profiling functions , etc..
I really suggest to take a look at it!
Both Windows and Linux versions are available for download
http://www.xdebug.org