CodeIgniter is PHP based an Application Development Framework and design pattern is Model-View-Controller (MVC). If people want to build website in PHP then CodeIgniter is the best framework. you can develop projects much faster than you could if you were writing code from scratch, by providing a rich set of libraries and helpers for commonly needed tasks, as well as a simple interface and logical structure to access these libraries.
CodeIgniter setup is very easy and faster. You can download CodeIgniter from official website codeigniter.com. It is providing very easy documentation. People can easy to learn this framework.
Before start Codeigniter, people should have knowledge of core PHP and HTML.
Tutorial available on YouTube.