On the profeshunl newbie blog there's a recent post looking at a database technology that's growing more and more in popularity all the time - MongoDB - and how you can use it in PHP as a data repository. MongoDB is one of a few popular NoSQL databases out there.
In NoSQL there are basically three kinds of databases; column oriented, key-value pairs and document oriented. MongoDB employs the latter kind. A document in this context is a structure of data with a given number of properties. These properties can be strings, numbers, arrays or objects, etc. If you've dabbled with associative arrays or objects in PHP, you'll know what this means. You can also group documents in collections. Also, there are something called sub-documents which are pretty much what you'd think they'd be.
He shows how to get the PECL module installed (three or four commands, ideally) and connect to your already-running MongoDB instance. He shows you how to create and insert some dummy data for DVD information and how to run an update, fetch and delete on the current data. There's also a quick mention of working with indexes on the data to optimize your results.