I had to use MockObject support to mimic NuSoap's web services functionality, which led to pretty loosely coupled components, which wouldn't have been the way I designed the project had I not used TDD.
Most people seem to be under the impression unit testing only works or is only needed for large projects. Well my project was pretty small (under 500 lines of PHP total) however it had high business value. It is a web services project that will be expanded in the future and will be dealing with millions of dollars worth of data. So from my view Test Driven Development can be applied for any size project if you will be adding on to it in the future or it has a high business value. I could at any point implement a new feature without fear of breaking something else down the road. If I realize my design sucks next year, I can refactor to a better design while the tests guide me to make sure I'm on the right track.
Special thanks to Marcus Baker, Jason Sweat and McGruff for the tips that helped me steer smoothly along the trail.