Nigel
I know that you can do this on a directory
by directory basis...
<Location /directory_name>
ForceType application/x-httpd-php
</Location>
This will force everything under the directory that you use to be parsed by PHP. This will, at least, give you some control over it.
I swore that I saw a way to do it exactly as you are thinking once, but I don't recall where.
Hope this helps you some, however.
Joseph