I write a lot of APIs, and I also preach that your API isn't finished until it has excellent documentation. Which is great, but that means I therefore have to lead by example and document my APIs :) Enter iodocs from the talented folk at Mashery.
Iodocs is a node.js application (which is fun for a PHP developer. Most developers write a bit of JS, but this one hasn't). You describe your API and all its methods in JSON, and then iodocs presents an interface for you to enter API keys, add parameters to each request and press the "try it!" button. This makes your API call and shows you the results on screen, which seems like a great way to demonstrate what all the various parameters do!