How to do a Smart cache...

Helloo PHP developers!

I wonder is someone has done, or how to it efficiently. I need a smart cache that can manager vars that changes frequently.

Let me explain it by examples, Imagine a site like www.digg.com, I want to cache the whole site, and at the same time just changes portions of it, like when a user vote, with out rebuild the whole cache.

This solution could offload the load of server because the database could only be used for store values, and let 'smart cache' to handle variables.

I want to focus on performance, so, If no one has do something like this before, what do you think is the better way to store variables?
- Flat file
- Shared memory
- Other??

Thanks to all.