CodeIgniter is a free PHP development framework. A commercial version named ExpressionEngine is available from the website.
Code Igniter is an Application Development Framework - a toolkit - for people who build web sites using PHP. Its goal is to enable you to develop projects must faster than you could if you were writing code from scratch, by providing a rich set of libraries for commonly needed tasks, as well as a simple interface and logical structure to access these libraries. Code Igniter lets you creatively focus on your project by minimizing the amount of code needed for a given task.