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 :)