CodeIgniter comes in three flavors: CodeIgniter 3 (current), CodeIgniter 4 (future) and CodeIgniter 2 (legacy)
CodeIgniter 3.xCodeIgniter 3.1.5 is the current version of the framework.
There have been a number of refinements since version 2.x, notably with the database, session handling and encryption. Development of this version is ongoing.
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CodeIgniter 2.xCodeIgniter 2.2.6 is the legacy version of the framework.
The 2.x branch was originally released January 2011, the next major update (2.1.0) came in the fall of 2011, version (2.2.0) came out in July 2014, and the current 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.
We encourage you to upgrade to CodeIgniter 3.x!
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