Download and install the PHP Apache Module provided by Marc Liyanage

http://www.entropy.ch/software/macosx/php

This package comes in handy if you want to use some advanced features like ZipArchive, XSLT, curl, etc
Apples default PHP  installation lacks all these useful extensions.
If you don’t want to spend time compiling PHP first, this package is a great time saver.

This does not seem to work with snow leopard.