Tool to develop OOP?

Just wondering (PHP), what tools do you use to begin designing your objects/classes?

We have a major class that we would like to deprecate and rework from the ground up but definitely want to plan it out first to ensure it meets all of our needs (and wants!)

Have you tried UML?

We are using Poseidon for UML 8, its very lightweight and at times I do miss the functionality that the more expensive versions have, but so far its not been mission critical.
http://www.gentleware.com/new-poseidon-for-uml-8-0.html

I am certain there is also open source / free software applications for UML as well if you take a look.