Writing an API (for mobile app) on top of a web application?

I have built my web application on Codeigniter 3. Now, I want to build mobile apps on top of it.

What's the best way to structure Codeigniter to do this?

I am thinking of having an "api" folder within the controller folder.

Anyone has experience doing this?

How are you building your mobile app?

Codeigniter is simply one of the tools you need to learn to be a successful developer. Always add more tools to your coding arsenal!