Ilia Alshanetsky has a new post to his blog sharing a method for fallback with PDO when your database connection fails.
For our database connections we PDO at work and we've extended the class with PHP to offer some other convenience functionality and wrappers. One of the things I wanted to do recently is allow the constructor of the PDO class to fail-over to our backup database connection pool in the event the primary was not available.
He wanted his code to catch an exception if the object was created with a failed server host and to connect to the backup instead. PDO doesn't cooperate with this method and just destroys the object when there's a failure. His work around is in this patch to the PDO support in PHP to provide a new constant, PDO:: ATTR_KEEP_CLASS_CONN_FAILURE, that tells the script wether or not to destroy the PDO object when there's a problem.