Yesterday I started porting Popoon to PHP 5. Usually it isn't a big deal to run PHP 4 application with PHP 5. This blog for example or the phpug.ch website run without problems with PHP 5. But as soon as you used DOM functionality, you're in deep shit ;)
Nothing works anymore. And of course, Popoon uses a lot of DOM and libxslt stuff. But it's not thaaat much of a deal to port it, it's just about function and method-calls renaming. Not a very creative job ;)
To make the transition as smooth as possible, I started also writing tests for Popoon. Being able to use the same sitemaps as before is the main goal for Popoon in PHP5. I used the traditional PHP testing framework for that. Not because it's fantastically good, but I'm used to it and it proved its usefullness.
All the other nice new PHP 5 features will then be later integrated into Popoon. As Popoon is highly OO, they will certainly make a nice addition.
Everything is in our CVS, so if anyone is interested in helping out, you know where to get it.