Just wondering if there was a way to have a php include without changing the file extension of a webpaage from .html to .php? The site I'm working on has already been established for a very long time, and changing the extension of files will cause utter chaos with backlinks/sitelinks/etc.

I want to implement php includes for the header/footer/etc. so they don't have to update every single webpage when they want to make change. If this is not possible with a php include, is there a way to acheive the same effect using different method?