Is there a way to save a page created in Designer MX 8 as a .php rather than .htm?

I want to incorporate a neat lil commenting script in my design and it "has" to be called from a .php page. I've tried to iframe it (as a widget) to no avail. I can create it and/or rename it from an html, but this would involve editing every page's navigation bar manually after final edits in Xara. A real PITA as you can imagine!! Can we select the extension name in MX's page naming properties? Below is the script's link and description...... it's FREE and works really well once set up. I want to use it to allow clients to post testimonials live.

thanks in advance.......

http://www.plohni.com/wb/content/php...ent_system.php

About "Advanced comment system"
This script provides an advanced comment system you can include in any of your PHP pages easily. The scripts needs to be installed only once and can be used on many different pages. It is very similar to the comment system which is in use on this website.