Why do you reckon its using a PHP backend and combination of Javascript/HTML[5]/CSS[n] for the frontend? Is that meaningful/helpful for something in particular? Could it be done differently?
Generally speaking, there are two functional areas: the front end (client/browser), and the backend (server/services).
The front end (and this is all pretty high level omitting all sorts of modern architectures) is the code that is consumed, parsed and [potentially] executed by your browser. That's the markup and style (HTML, CSS), the browser side (aka, client) Javascript. All this runs in the browser, you don't need any server/services, you can write all this in a text editor, on your desktop, open the file in your browser, and you'll get a formatted web page, with possibly features like menus, image carousels, etc.
The backend, provides services that run on specific applications, these are things like web servers, different language runtimes on those servers, databases, file ("object") storage, etc. If a site has a need to store information in a database (for example, a list of products for sale), then you need the database server, a web server, and some language (slightly more complex but you get the idea).
Ultimately, what these backend services do it render out a web page:
[front end: YOUR BROWSER makes a request to a website] >> [back end: DB >> WEB >> GENERATED PAGE (HTML/CSS/JS)] >> [front end: YOUR BROWSER consumes supplied HTML]
The page itself is (for this discuss) no different than the HTML doc you can create in a text editor on your desktop, it's just sent across a network (using the HTTP
protocol).
So pretty understandable, right?
So to answer your question: if the site needed some kind of backend services, for example, content was stored in a DB, then there needed to be some sort of DB + Web Server + Language. They decided to use PHP as the language (running on a Linux server running the Apache web server and a mySQL database).
However, they could use dozens of different options on the backend to achieve the same design. Instead of PHP, they could use Python, Ruby (with a web framework), they could use Postgres for the DB, it could be implemented using MS based products like ASP.NET and MS SQL Server, etc, etc.
So bottom line, there are dozens of ways to achieve those backend services when they become necessary, but they all serve up a common set of browser consumable content.