It’s been along time since I did that with Dreamweaver (I no longer use it). However, I think you have to look at the config files and can’t do it from within Dreamweaver itself.
I think there is a file called Extensions.txt, if I remember right, that holds that data. Bascially, it says which extensions to process as which type of pages. It might be, MMDocumentTypes.xml, though.
Sorry I can’t be of more help, but hopefully that will get you started.
Just to be clear, Dreamweaver won’t actually process the PHP. It will just display it with coloring, formatting, etc. Also, in design view it will show you the variables, but it won’t show you the RESULT of the variables or any of the finished product.