CodeIgniter 4 is the upcoming version of the framework, intended for use with PHP 7.2.
Development is underway, and the current version is v4.1.1.
You *could* download the V4 framework using the button below, but we encourage you to check the Installation section of the User Guide, to choose from several different ways you can install the framework :)
CodeIgniter 3.1.11 is the current version of the framework, intended for use with PHP 5.6+.
There have been a number of refinements since version 2, notably with the database, session handling and encryption. Development of this version is ongoing.
CodeIgniter 2.2.6 is the legacy version of the framework.
The 2.x branch was originally released January 2011, and the last version (2.2.6) came out in October, 2015.
CodeIgniter 2 has reached its end-of-life for support and updates, as of October 31, 2015. No further updates are planned.