PHP Engines was a contribution from community members and it is pretty awesome compared to what we used to have. That being said, it has a scalability issue in its design, obviously. Nick's workaround should work well. We will be working on these over the next few days but the app itself needs a bit of a rewrite.
Currently the app requires the php versions. It should be more liberal and run even if no engines are installed. The engines themselves should be app-ified and placed in the marketplace. They should require php-engines but not the other way around. That way we can poll from the marketplace which ones are available for install and can poll from the installed list. This way it is open ended and the app shouldn't care what is installed or not installed from a dependency standpoint.