Kohana is also a good choice. Initially seconded by CodeIgniter, it has now been rewritten as a real open source PHP5 project that is optimized for speed, rapid development, and flexibility.
Lateral note / disclosure, I was initially involved in a fork that formed in Cohan. CodeIgniter (at least about a year or two ago when I was involved) is not very open to user contributions. I had several improvements and bug fixes in which I made corrections that ended up rotting as they were ignored *. As a result, several people showed great interest in their contribution, and while Kohana is based on all the same principles and ideas as CodeIgniter, the implementation is much better and much easier to work with.
(*: not because of the quality of the code, just to simply not be interested, because it did not necessarily correspond to the direct direction of Ellislabs, because it was support for their commercial product. Even if he made a great contribution to the library, it would be required, so that they rewrite / modify everything that they built on top, this would be ignored.)
gregmac
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