Okay, so I figured it out (I think):
If you look on the bar that has:
[<>] Code | [--] Split | [] Design
you'll see a button right before "Title" that has a lightning bolt. That's "Live Preview". If you click that, it should run your PHP code for you automatically, or on refresh. Now, this will run all of your PHP code. It's probably not what you expect, but it's close!!
You may have to define a site to use with it, but it's a step in the right direction.