PHP is a traditional server-side language, meaning it offers a lot of ways to interface with server software like databases and such. Because of that, it is not sensible to implement PHP as a client-side language in the SAFE browser. Your confusion arises from this fact, I think.
So, when you would want to use PHP, it will be outside of the SAFE browser (similar to e.g. a NodeJS application). Indeed, as PHP is usually run on the server, you will probably still want to run it on a server. In that case a logical example of an application is given by @intrz. Your PHP server could still be accessible on the ‘regular internet’, and could communicate with the SAFE network internally. As such, it could form a ‘bridge’ between both networks.