Hi all,
New here, Not very experienced with PHP.
I've got version 18 of nusphere. I have a program called osCommerce which is 100% PHP. I need to convert several html5 documents to PHP. These documents will be called by a template for inclusion in a different document. Unfortunately they are HTML5.

Can anyone suggest a quick and dirty way to convert them to PHP? All help will be appreciated.

There is no quick and dirty ways to convert an output into a source code that generated this output, unless you're fine with trivial case that would just echo what you need, but it's probably not the case, right?
Probably you need to be able to update some parts of that HTML. In this case you need to convert it into template, not php.
If so, you need to learn osCommerce templates and learn how to invoke them from the other scripts.