Codeigniter using namespace for version 3.x?

You can use namespaces in your application, but CI itself doesn't use them, and its loader doesn't support them, so you'll have to load namespaced classes on your own. Since CI will load Composer for you if you set a config value, it's usually pretty easy to load and use namespaced classes as you would in other frameworks.

Bonfire

Practical CodeIgniter 3
CodeIgniter Testing Guide