Since I spend most of my days programming PHP I tend to need that a lot and a few years back I wrote a small script that would let me type php code in my browser and execute it. Nothing fancy, but quite useful. Over the years a few people got interest seeing me use it and asked for the sources, so instead of repackaging it every time, I thought I'd clean it up, polish a bit, add some features, and put it on github.
Setup is as simple as dropping the code somewhere in your local server's document root and offers a textarea for input and a expandable tree for the resulting output. It uses the Krumo tool to create this modified output. You can see a screenshot here.