Oh, I see what you mean. I thought you meant output HTML and templating vs server side logic, but yes, obviously I know the difference as the only front-end language is JavaScript and derivatives.
I’m more used to the terms ‘server-side’ and ‘client-side’ btw. Obviously PHP is server-side. But not all server-side is backend in my view. I consider backend to be application logic and things like database access. Templating can be done on the server or the client, and that’s front-end.