If you're relatively new to PHP development or have just meaning to get into using relational databases with your application, you should check out this new post from NetTuts giving a "for dummies" approach to learning about RDBMSes.

Web apps can be split into two major components: a front-end that displays and collects information, and a back-end for storing the information. In this article, I’ll demonstrate what a relational database is, and how to properly design your database to store your app’s information.

They describe the usage of a database in several steps (no code is included, just SQL at the end):

• Get Some Data
• Remove Repetitive Data Across Columns
• Remove Repetitive Data Across Rows
• Linking Tables with Keys

This is followed by links to a few of the different RDBMSes out there and a brief introduction to what SQL is and what it looks like.