persistent storage in PHP

How do I cache data persistently across page calls? As near as I can tell all the variable scopes in PHP terminate at the end of a page and the next page starts with a clean slate. Surely that's not 100% true and there's some way to store data that will be available on the next invocation? Perhaps there's some external shared library solution that stores data conveniently? I know I could write out a file and then load it back next time but if I do that I may as well just go back to the database. I'm going to have tons of itty bitty bits of data all coming from the database and I want to just cache in an associative array to reduce traffic since they're infrequently updated data. -- greg