File extension doesn't make a difference when it looks like this:
page-name.html and page-name.php

however it makes a difference when it's like this
page-name.php?page=name&id=8&product=product-name

because some search engines cannot understand this url. So in this case it would be best to change it to something like
id-product-name.html
or id-product-name.php (doesn't matter which)

and yes <h1> can make a difference <p> cannot. Just don't overuse it.