PHP 5.3 shipped with a new driver for MySQL, mysqlnd or MySQL Native Driver. While one of it's biggest features is that it gives us a mysql driver that is part of PHP so it doesn't suffer from licensing issues that the previous driver did. However, in the redesign, the architecture was changed.

He mentions the mysqlnd_uh that's a part of this new MySQL driver and an interesting article from Ulf Wendel about its use in transparent load balancing and sharding. The article includes code that randomly picks a MySQL server for the script to use.

The article is a bit difficult to read at time but trust me, stick with them because the stuff in there is the stuff you want in your head when you are working on your next application.