Your question is unclear.
You just want to change or hide extension of file on website?
Changing a language like this would be like using Google translate - totally useless, unless you had a good grasp of both languages to begin with.
However, you want to change the extension from html to php. Why?
I think WP uses templates (not having much use for it, I'm not sure), and if so, the html page will have html (obviously) with variable placeholders. You can't or shouldn't include php in these files. There will be other files that pass (assign) data to them. These are the ones you need to change to insert your php. You then add some html and placeholders to the html file. At least that's what usually happens with other template-driven sites.