You can configure your webserver to let the php engine process the ".html" files, but not many people would do that. You're better if you keep php code restricted to ".php" files.
If you want to do this for
seo purposes, then it is possible for you to achieve this effect via URL rewriting. With URL rewriting, you can tell your server to serve "foo.php" when a browser requests "foo.html", or even "bar.html". You need to research URL rewriting.