Is it possible for the preview to pick up the export file type? I have several pages that have .php extensions on the export; when I preview them, the .php is not exported in preview mode, therefore the browser rendering is incorrect.
Maybe I am not entirely understanding you but when I preview a page that has a php extension by clicking on the little globe in DHE it opens in the browser and I can see the lay-out fine.
You are understanding me... I have .php as my export type extension, when I do a preview, the extension in the browser address bar is .html, not .php (..preview.htm)
This is normal. As soon as you publish your site or a page then it will have the php extension. You will then need a server (local on your machine) or on the net to view the page.
DHE can't preview the page when it has a php extension because this is a server side script that needs a server and all it's extra's (apache) to be viewable so it temporarely turns it to htm(l) just for previewing purposes.
The "preview page" function creates an HTML page and invokes it directly. A .php page requires a WEB server in order to run it so it previews pages creating a .htm copy of them as Martin said ;-)