The Zend Developer Zone has information about a case study as performed by IBM and Zend showing the performance of the Zend Core for IBM (DB2 9) to services - you ready for this? - 10,000 active database connections.
The PHP application used for this study is a DVD store application that simulates users logging into an online catalog, browsing the catalog, and making DVD purchases.
The solution presented in this paper demonstrates how Zend Core for IBM with IBM DB2 9 performs, scales, and offers the best total cost of ownership. This is showcased by scaling-out Web servers as users to the online store increase. The single DB2 data server manages this without requiring the addition of CPU and memory using its unique connection concentrator feature, which manages and balances the workload on the data server. This solution is able to efficiently service tens of thousands of simultaneous connections to the database using this powerful feature.
They've included complete information on all of the testing and infrastructure they used to achieve this impressive feat. There's even graphs and images for those skimmers out there looking to glean what they can.