I've build several php sites, but so far all of them was aimed at being hosted on a single server (shared and dedicated alike).

I'm curious about how one should go ahead to build a php application that can be mirrored across servers (globally).

For instance, one thing that comes to my mind is that one cannot be sure the next request from a user will be served by the same server. So session won't be working (I've read somewhere one can have a session server though, but let's assume that won't be available).

Any readings, articles, tips and pointers on this?