Simas Toleikis has posted some caching techniques he's come up with to handle a few different situations including simple file-level caching and working with memcached.
Caching intermediate data is the most primitive and yet the most rewarding optimization technique where you don't need to fiddle with any complex algorithms or premature optimizations. If you are into PHP you should be aware of all the best practices and the right tools for this job to make your websites fly.
He covers a few different, though common, situations you may come across in your application where caching could be very helpful:
- Static scope variables
- APC shared memory functions
- Memcached for large distributed caches
- In-memory database tables
- Simple file-level caches