Justin Silverton has posted this introduction to using SQLite and PHP on his blog. It describes what SQLite is and how to install PHP support for it.

SQLite is a small library that implements a self-contained, embeddable, zero-configuration SQL database engine. Features include: Transactions are atomic, consistent, isolated, and durable, Zero-configuration, Database files can be freely shared, Supports databases up to 2 terabytes, and many more.

Why would I need this? If you want to have the advantages of a SQL database, without having to install a separate system such as mysql or postgres. It has also been demonstrated to work on websites getting more than 1,000,000 hits per day.

The installation section is a bit short on the details, but does briefly cover both the unix and windows sides. Also included is a brief example of how to make a query on a SQLite database via the PHP functions compiled in.