Codeigniter is a MVC framework in PHP, It is used to build a dynamic websites in an interactive way. Model-View-Controller is an architectural pattern for implementing user interfaces. Model will interact with database, view is the front-end and the controller is key feature this will act as mediator between the model and view.

• Simple and light weighted
• Easy installation
• Inbuilt libraries

This framework has many built-in libraries and functions, you can use those libraries(like email, paging and etc) without any additional configuration. Note: Don't use Codeigniter framework for smaller application.Because this framework is made to build for medium to large applications.
What are you waiting for, Lets get started with Codeigniter.