From the International PHP Magazine website:
Have you ever wanted to write a Web application that truly seems like a desktop application and has its functionality too? And did the tons of CSS youâ€™d have to do scare you? Gecko-based browsers are awesome to render pages properly and are compatible with the latest standards. You can use XUL's magic to make a Web application look exactly like its desktop alternate. The article explains XUL and how to use PEAR's XUL package to generate XUL pages on the fly.
Be sure to check the rest of the issue too - there's articles on Ajax hype, working with REST, security tools, and the continuation of a beginner's series on PHP.