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However, you do not need to change the file extension to use includes - there are other options. See: [webmasterworld.com...]
I use PHP a lot, and there's also a way to tell the server that even though the filename ends .html to still go and look inside it for PHP code. With that method, you don't need the rewrite at all.
So, there's at least two ways to achieve this; neither of which see you having to change the URLs that users 'see' and 'use'. That is always a better method than changing URLs, with all the attendant ranking and traffic losses.
See also "Cool URIs don't change" by Sir Tim Berners-Lee.