CodeIgniter is a great framework developed by EllisLabs
While most of open source projects are lack of documentation, CodeIgniter documentation is first class. EllisLabs puts an extra effort to give quality documentation to the community.
Either you're a beginner or advanced programmer, you'll be impressed by this documentation.
CodeIgniter follows MVC architecture. It will help you to layer your application, so you won't end up with spageti code. More on Model-View-Controller.
With database abstraction, you don't need to worry when you switch to other database engine.