This is a quite wide question, and it's going to be pretty difficult to give you a definite answer -- but a couple of ideas :
- First of all, do not pre-optimize !
- Make sure your application works ; that's the most important thing.
- And, only when it becomes necessary, start optimizing.
- PHP by itself generally scales well :
- Add a couple more Apache+PHP servers, load-balance your users
- And this tends to work really easily
- Filesystem doesn't scale that well / that easily :
- it's not shared accross servers
- sharing filesystem (with NFS, for instance) can sometimes cause problems.
- The database is generally the hardest part, when it comes to scaling :
- Having more than one "write" server is hard
- Having more than a couple of "read" servers, generally using replication, can become a pain for maintenance
- You'll have to think about sharding, one day or another, if replication is not enough.
- Use lots of caching : the more cache you can use, the less queries you'll make to the DB, the better it'll be
- memcached is great, and scales well : just add a couple of servers, and you get a couple of more GB or memory in your caching-cluster
- Using a reverse-proxy, so your Apache+PHP servers have less work to do, helps too.
And a quick couple of links that might give you some ideas :