In your httpd.conf file, find your PHP AddType
AddType application/x-httpd-php .php
Change it to something like
AddType application/x-httpd-php .php .htm .html
That should result in anything with those three extensions executing as php and that is what I would do.
Two other ways might work for you too. You could use mod_rewrite to rewrite all *.html urls to *.php and then rename all .html files (I realize that's not what you asked, but it would mean that any links in Google or whatever would remain valid and your result would be the same).
You can also try the ForceType directive which will force anything to execute as the given type. I paste this in from the Apache manual
ForceType directive
Syntax: ForceType media-type
Context: directory, .htaccess
Status: Base
Module: mod_mime
Compatibility: ForceType is only available in Apache 1.1 and later.
When placed into an .htaccess file or a <Directory> or <Location> section, this directive forces all matching files to be served as the content type given by media type. For example, if you had a directory full of GIF files, but did not want to label them all with ".gif", you might want to use:
ForceType image/gif
Note that this will override any filename extensions that might determine the media type.
