On the "PHP in Action" blog, Dagfinn Reiersol has posted some ideas about developing PHP applications (really, any applications) from scratch - by faking it.
So where and how on earth do you start? Let's say we know one thing about the result; call it the known fact. Do we start by making educated guesses about what should be in the black box? A lot of development is like that.
Enough of this philosophical mumbo-jumbo; what does it mean in practice? Well, it's test-driven development, and more specifically, it's what Kent Beck calls the Fake It patttern. You implement a test for the result, then you hard code the result. It's an absolutely weird idea that's magical in its effect because it get you started. And as the German expression goes, aller Anfang ist schwer--all beginnings are difficult.
He illustrates the point with a set of code examples - one side being the unit test (written first, of course) and the other the actual code. All it does is set a URL to a variable inside a class, but the point is made.
Starting at the high level is good when you know the full scope of the project, but that's not always a possibility. This is a good alternative to getting started without causing too many issues down the road.