EDIT Nov 2018: Although I wouldn't knock CodeIgniter, nowadays Laravel (currently 5.5) is the framework I use.
Here is a good article that sums up the reasons I use Laravel.
To get jump started, I recommend Laracasts. It's a subscription video tutorial service that goes in depth on how to use Laravel (and other web dev related things).
Codeigniter, to me, is the easiest of the Rails-like frameworks. It's bare bones, and you can build a CRUD app from scratch easily.
The biggest issue with rolling your own app is security. Codeigniter can help you build a less hackable site by shielding you from many of the common security risks, such as using $_POST arrays directly, and not properly filtering your data. Not to mention the many helper classes it offers such as form validation.
You can view the documentation on their website. It's very easy to use as long as you remember the navigation is hidden at the top of each page. :D