Have a look at
Yii.
After reading up on frameworks I decided to use Codeigniter but after a bit more research I then intended to use Kohana v3 as it's based on PHP5 rather than PHP4, but when I started using it I found the lack of documentation a problem.
I then switched to Yii and found the documentation and guides to be pretty good, and any questions that came up had already been answered on the forums.
There's definatley a learning curve if you're coming from raw PHP into the MVC and OOP world.
Yii seems particulary good for database heavy apps as it has code generation for a lot of the repetative admin backend stuff.