A PHP file is
kind of like an HTML (web page) file. The difference is, HTML files contain all the content and can be viewed straight in your browser, usually from any location. For example, if I download an HTML file containing a string of text, my browser will be able to show me that text no matter where I put the file, because it is stored within the file. PHP files on the other hand
often (but
not always) only contain code which relies on the server to generate the content, based on the code some other resource, for example a database. If you look to the top of your browser whilst in PPRuNe, you will see that the site uses php to generate the various different parts of the forum. The PHP code can be used to manipulate a database of all the forum posts, or something similiar, in order to generate the pages we see.
The PHP file might still have information other than server-side code in it though, because they can contain HTML code just like any HTML file, so try opening it in your browser. Otherwise, the code in the file will just try to reference a server which doesn't actually exist (unless you have a server with a PHP module installed on your machine

).