On CoderWall.com there's a post sharing some findings about how RockThePost.com scaled PHP to 30,000 concurrent users on Amazon EC2 instances.

RockThePost.com is a LAMP stack hosted on Ec2. We're preparing to be featured in an email which will be sent to ~1M investors... all at the same time. For our 2 person engineering department, that meant we had to do a quick sanity check to see just how many people we can support concurrently.

They have a list of five suggestions/opinions on what has helped them scale out to this level including using Varnish for caching, turning on APC (PHP opcode caching) and using a c1.xlarge EC2 instance. They used Google Analytics and Siege to run their load testing on the "exterior" of the site.