It's more architecture than stack, IMO. If you focus on minimizing shared state, use caching well, and have a performant database backend, you can make pretty much any stack scale by just throwing more nodes on the front-end. Understanding load balancing and how to partition your app so that you can scale horizontally are crucial.

There's a pretty good article here: http://blog.nickbelhomme.com/php/scalable-web-applications_1...

that has some good stuff in it. Also, if you read over the material at http://highscalability.com you'll find a ton of into on this stuff.

Well, thinking of scalability at this young point is not recommended, as you might forget other true important things, like actually getting to the point where you need to worry about scalability :).

Just go with what you like to work with, because you will spend a lot of time doing that respective work, and working with something you don't like for the sake of something that may or may not happen.

I wont go into technical details as I am sure there are better out there to explain this, but story on short, don't worry everything can be scaled nowadays.

Forget about scaling, it does not matter.

(Unless you're building something that needs to scale day 1, say, an aggregator, a search engine, or something like that.)

Money.