Evolt.org has a recently posted tutorial that talks about creating a simple failover database connection for your PHP and MySQL application.

Imagine, you're connecting to a mySQL database with PHP, but hell, you want a bit more robustness. So why would you want just one database? You'd use a failover right? So if the first database is down or just doesn't connect, it can connect to the next one. Here's how to do that (I'm assuming you're already replicating between databases so that they're roughly in sync). Here's how you do it.

The tutorial walks you through the creation of a simple MySQL connection, but with two sets of credentials. The script tests the connection to the first database and, if it doesn't get a valid resource back, it falls back on the secondary. Of course, keeping the data in sync is your job.