I've installed the Google App Engine SDK for PHP:
Download and install Python 2.7
Download and install Google App Engine SDK 1.8.8
After those steps, I got an GAE launcher on my desktop. I clicked on it, it displayed a loading icon, and a few seconds later the icon changed back and nothing happened. The launcher seems not to work.
I'm using Windows 8.1 and have installed XAMPP before (hope this is not the problem).