The primary documentation for CodeIgniter is its User Guide.
The User Guide contains an introduction, tutorial, a number of "how to" guides,
and then reference documentation for the components that make up the framework.
CodeIgniter 4
CodeIgniter 4 is the upcoming version of the framework, licensed under the MIT License.
CodeIgniter 3
CodeIgniter 3 is the current version of the framework, licensed under the MIT License.