PHP is a processing language that runs on most webservers. It can be used to calculate stuff, read/write to databases, etc.
Typically PHP generates a HTML page by injecting a HTML template with calculated results, so most PHP pages are essentialy a mixture of static HTML code that is updated by the PHP processor to form a web page.
If you have no PHP processing content there’s no reason to use a PHP extension and it’s likely to make your web page render more slowly by forcing a request from the web server to process the page rather than serve it from any cached version in the browser or webserver cache.
If you do select php and there’s no PHP code present, then the page will just be served as-is with no changes made to it.