The old pages should not lose their PR and rankings if you do not change the URLs in any way, and do not change the textual content and HTML title tags.
You can create static HTML pages and name them with the PHP extension. They will get run through the PHP parser on your server, but there doesn't have to be any PHP code to execute.
As Aquarezz pointed out, there may be a folder with a design template. If not, then just use View > Source in your browser and cut-and-paste the code into a new page in your HTML editor. You have to do that manually for every page in the site. Just make sure that the URL structure is exactly the same when you set up the new site.
Another alternative is to buy a couple of books and learn some basic PHP. It is a relatively easy language to learn.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin