Jamie Rumbelow has put together a post that talks about building the perfect environment for running the CodeIgniter framework with test-driven development processes.

As CodeIgniter has little built-in test support, it's often difficult setting up a good environment to allow Test-driven Development using SimpleTest and the CodeIgniter framework. I've been using Ruby on Rails a lot recently, and I was inspired by the fantastic built in integration with unit, functional and integration tests, so when I returned to CodeIgniter, I delved into the deep dark world of setting up the perfect CodeIgniter and TDD environment.

He looks at a few different steps in the process - where to put the tests, making a test runner to work with them, hooking this into CodeIgniter and coming up with custom test classes/a GUI to make getting the results easier.