What is Codeigniter

CodeIgniter is one of the many PHP frameworks existing. It is an open source web application framework, use in building dynamic web sites based on Model-View-Controller development pattern. If you want a framework with a small footprint, with exceptional performance, and a framework that requires nearly zero configuration then Codeigniter is what you are looking for. It lets you focus more on your project design and it minimizes the amount of thorough coding for a given task.

The first released version of CodeIgniter was on February 28,2006 and currently they have the latest stable version released last November 1, 2011 which is CodeIgniter 2.1.0.
You can find list of versioning here(http://codeigniter.com/user_guide/installation/downloads.html), all source code is maintained at GitHub.


Server Requirements

• PHP version 5.1.6 or newer.
• Database supported: MySQL (4.1+), MySQLi, MS SQL, Postgres, Oracle, SQLite, and ODBC

Check the License Agreement here.

If you have been using lower version of CodeIgniter(like version 1.7.2) on your project, it is not merely advisable to update your CodeIgniter. There are big changes after they've released version 2.0.0. Check on the changelog for further information.

Credits
CodeIgniter was originally developed by Rick Ellis (CEO of EllisLab, Inc.). The framework was written for performance in the real world, with many of the class libraries, helpers, and sub-systems borrowed from the code-base of ExpressionEngine.

It is currently developed and maintained by the ExpressionEngine Development Team.
Bleeding edge development is spearheaded by the handpicked contributors of the Reactor Team.

A hat tip goes to Ruby on Rails for inspiring us to create a PHP framework, and for bringing frameworks into the general consciousness of the web community.