Kohana is my favorite PHP framework by a long shot. The developers are very bright and they work hard to incorporate the wants and needs of the community, without diluting quality. It is PHP5 only, though.
Use CodeIgniter if you need PHP4. The two are similarly flexible and light.
++ for KohanaPHP. It's one of the most elegant and powerful PHP5 frameworks you will find out there. The devs are great!
Positives:
* cascading filesystem with built-in support for modules
* very clean and easy to use ORM implementation
* auto loading of classes
* easy to understand syntax
* HMVC support is currently in development
* open communication between community and devs
* flexibility ... you can pretty much do whatever you want with custom class libraries, vendors without much hassle.
Negatives:
* still a relatively new framework that is changing fast, but the current trunk branch is now stable.
* expanded documentation module is in development
I had a look at Kohana after your posts, and I really like the look of it, especially since I was leaning toward CI however didn't want to learn in an environment which was supporting a mix of PHP4 and 5. If I'm learning PHP now, I may as well learn PHP5 and work in an environment which can fully facilitate that without having to bloat up and lose flexibility by supporting 4 as well.
Kohana really does look like a great framework, however after reading the forums and wiki a little I'm worried that if I begin learning now, I'll need to change a lot of what I learn in the coming months with the 2.3/3.0 branch (whatever they end up calling it) - and whatever I start building will have to change too.
This isn't a huge problem as whatever I build now will be mainly side-projects which are designed to push my learning forward (such as small e-commerce sites, rating sites, file uploader sites), however considering the changes coming soon (API etc) it's a little frustrating.
However, thanks to you two I've found Kohana which is absolutely what I was looking for. Cake is a little too bloated and complex, CI is more flexible and simple, however to appeal to a larger userbase must support also PHP4 and 5, while Kohana is very niche insofar that is all about 5 and thus very much less restricted.
I'd be interested to see what other people who have worked around these frameworks think of Kohana.
Use CodeIgniter if you need PHP4. The two are similarly flexible and light.