Open Source PHP Framework (originally from EllisLab)
For more info, please refer to the user-guide at http://www.codeigniter.com/userguide2/
(also available within the download package for offline use)
WARNING: CodeIgniter 2.x is no longer under development and only receives security patches until October 31st, 2015. Please update your installation to the latest CodeIgniter 3.x version available (upgrade instructions here).