Read/Process Included PHP files in Preview

I would love to have my included PHP files render when I preview a page.  I keep my header, navigation, and footer content in separate pages.  When I am working on a content page, I include these files to make development more modular.  However, when I preview a page with the includes, only the content in the active page renders, not the included content.  Can this functionality be added?  Thanks for an amazing product! :)