SIFO is a well-proven open source PHP5 framework currently running on several sites, from small installations to large websites with multiple servers.
SIFO is free and very easy to use. Contains a lot of libraries and classes that resolve your basic problems and allow you to focus in your business logic, at a very low learning curve. If there is a curve :)
It comes with several technologies ready to use such as Memcached, Sphinx, Mysql, Redis... and all the debugging tools.
You can read more documentation in the official site: sifo.me