This is not really a talk about the differences between PHP 5.3 and 5.5 – the PHP manual’s migration guide has that angle covered pretty well. This is a talk about why we decided it was time to upgrade. It’s about how we managed to safely upgrade our large codebase. It’s about how we minimised disruption to the 20+ person development team. It is a talk about what happened when we rolled out the changes to over 100 machines across two data centres. I’ll be talking about some of the things we’d done that made this sort of thing relatively “easy”. However, it didn’t really go quite as smoothly as we would have hoped. I’ll be sharing stories about the problems that arose in production: despite our best efforts there are still differences from the development environment. PHP 5.3 is down to security updates only now, so if you haven’t upgraded yet you will probably need to soon. Hopefully our experiences can help you get there, and avoid the problems we ran into.