As far as I can tell, there are no RRDTool bindings for php5 for Ubuntu 9.04. It appears as though it existed in prior distributions that included php4, but Ubuntu 9.04 no longer includes php4.
Several others are talking about compiling it from source, but this seems very wrong and unnecessary to me.
If you have any other information about how to easily via apt-get make RRDTool functions work in php in Ubuntu 9.04, please leave a comment here.
There’s not really a easy/good way to install it, but it’s definitely doable. Unfortunately it involves compiling from source though, as you mentioned (eek). Here’s a little guide I wrote up on it a few days ago. Maybe it’ll help, if it’s not too late?
http://thirdroute.com/installing-rrdtool-for-php-on-ubuntu-server/
Anyways, just ran across your post today and thought, “Hey, I can relate!” If you have any questions, fire an email or comment my way.