Web application often will require a database. PHP works well with MySQL database. That is why many PHP applications (such as ZenCart, Wordpress, Joomla, Drupal) uses MySQL as the database backend. Here are some tutorials on learning how to have PHP interact with MySQL database.

• Connecting to MySQL using PHP Video by KillerPHP.com
• PHP Basic Database Handling on phpfreaks.com
• MySQL's Insert on Duplicate Key Update Syntax - this is useful MySQL feature that converts an insert to an update when trying to insert a record with a duplicate key. For solutions for other database, see database programmer blogspot.