Shay Ben Moshe's Blog: PDO Persistent Connection Analysis
PDO is an abstraction layer for database connections in PHP, and it became increasingly popular in the past few years. PDO gives us the option to use a persistent connection. If we don't use this option, a new connection is created for each request. If we do use this option, the connection is not closed at the end of the script, and it is then re-used by other script requests.
He shares the setup for his testing (hardware and MySQL configuration) and some of the results from his tests using the Apache ab tool for making web requests against an application. You'll need to download the archived file to see the results, though. It also includes the files he used to test with, comparing regular connections to the persistent ones.