Sorry for the confusion there, I went ahead and clarified the descriptions.

Tandem runs any application, but needs the appropriate extensions in order to write code. It has the capabilities of writing PHP, but that isn't implemented yet.

The code writing aspect of Tandem is really just a small feature of the whole application - The focus of Tandem is really to provide a preview (or many previews) of your application with tooling that makes it easier to fix visual bugs.