Hi,
HTML files are simple files which can determine the content and layout of a page. They only contain fixed content (unless you start getting fancy with JavaScript).
PHP files are usually used to generate the content on a web server, so you can't simply open them up on your computer. For a quick and easy way to set up some PHP software on your computer, check out
XAMPP (it's what I use on Mac for development). For ease of use, I'd recommend
not getting any of the "VM" downloads, so as of the time of writing this post the version I'd get is "7.3.1 / PHP 7.3.1".
Using XAMPP, in your Applications -> XAMPP -> htdocs folder, you can add the Nameless files, boot up "Apache" and "MySQL" within XAMPP, and then visit
localhost in your browser to view the site.
Hope that helps :)