what is the concept for clustering web apps?

I am going to built a web apps that (hopefully) someday it could serve thousand or may be millions users.

Now, I need to know concept of clustering web apps. I have no experience about this in the past.

For example, I have 2 servers, one located in Australia and the other located in USA. Both will be a cluster so they share the same database but just different server. If the user comes from australia I would 开发者_运维问答like to use australia server but if not from australia we will redirect to US server.

could I make that using PHP ? I am going to use Kohana or Yii (still doing feasible test at the moment).

any good books that explain about clustering web apps ?

---

In a such scenario you would use a load-balancing cluster. On wikipedia there's a good overview about clustering in computing. To realize a such scenario you would normally do that with a load balancer (hardware).