The extension depends on your web host. I’ve got my sites on three servers. One lets php code display on .html files, and one has a control panel widget that let me add extensions which allow php, so on those my extension is always .html

On the third server, I use .php, because that’s the only extension they allow for running the code. There might be a way to get around it with an .htaccess file, but I’ve never bothered to try since, in my experience, Google isn’t particularly biased against any page extension as long as the page is static.