I am guessing when you say "just change the extension to php" that you do have PHP enabled on your server, right? Essentially yes, you can enable PHP that way on a page. PowerCMS will generate PHP statements to add into your page... so you will have to be able to do that. You will also have to run a provided sql script on your database and set up a handful of parameters (I think about 6) on the global variable page and then you are ready to go.
Cheers,
Brian