What is the easiest way to learn CodeIgniter?

I definitely would suggest learning both CI and Laravel.

I would start with CI as its the most easiest to learn, when you get footing on the principles, you can start looking onto laravel.

But i would say CI3/4 is easier and has a small learning curve compared to Laravel.

When i was trying laravel. They have upgraded to a different approach making my effort in vain. 

So i dropped laravel and went with CI.

But time after time i would take a peek at laravel docs and try the framework.

You never know,