my question is if there is any reason at all not to just convert all of my HTML files into PHP files so that I can put PHP code into them. Are there some cases where naming a file *.php will make the HTML parse incorrectly?
So is there any point in keeping HTML documents at all, then?
You have to think it the other way round. There should be a reason to convert HTML to PHP. If it doesn’t exist, then don’t do it.
If you do, there would be more “latency” between the user request and the web page render, because of the interpretation of the PHP server of your page. Additionally, you add complexity to your system (server configuration, security fixes, etc).