On PHPFreaks.com there's a new tutorial posted to introduce you to using MongoDB with your PHP applications.

Most of us grew up on the concept of LAMP(Linux apache mysql and php) but there are times when that basic setup can become a real pain to maintain and scale. Generally the hardest part to scale out of a LAMP setup is MySQL. When running a large site with lots of data and traffic optimizing/scaling MySQL can be a really complicated undertaking. That is where NoSQL comes into play.

You'll need to have a Mongo instance installed to follow along with some of the examples but that's simple enough with the help of a package manager. They show you how to create the connection, insert documents, query them, delete them and work with GridFS to split up files across multiple documents.