I created my web page and it has grown over time. When I originally made the page, I didn't create the PHP function for my header and footer and since I didn't want to spend the time to fix it, I just left it.
I have finally got to the point where wanted to add the PHP Include function which worked perfectly except it seems that I have to change all my pages from .html to .php.
What will that do to all the SEO work I have done over time? Is there a better way to do this?