Not for 'how-to' coding questions but PHP theory instead, this forum is here for those of us who wish to learn about design aspects of programming with PHP.
Doubt there is one, in my mind at least. It's an application/system spec, ergo.. the terminology is flipped round and you don't have a System under Test, you have a Specification for a System.
You won't regret reading it, or writing tests / executable specifications for that matter. The author has some Monk like patience to write in the amount of detail he did but it is excellent. Particularly good is when he talks about observations points vs measuring outbound behavior. Keep in mind if there is an observation point it "guides" you into having the right accessors in your API. Also note observation points and TDD aren't exclusive, you still have observation points with BDD which was another point of confusion for me when I started out. I thought BDD = mockist but it doesn't necessarily.
Even if I was doing BDD ( which I don't ) I'd still call them SUTs and DOCs just to use standard terms. If you are doing BDD you're already doing TDD, you're just doing it in a more human compatable way. in my opinion