Themes

I am looking for the bit of code that calls the header.html file, without success. Does anyone know where the header.html is first called?

I am trying to track it down because I want to create a theme that is solely based on XUL and PHP. In order to do that I'd like to change header.html into header.php. I need to know what conflicts I'll create by doing that.

If I were to put my PHP and XUL directly into the header.html file, I don't think it would render everything correctly.  I try this theory out. 

Chris