A local licencing system?

Hello everybody,
It's been a while since I started using CodeIgniter and finally I made an app that would be really useful. The last step on my process is to add a licensing system to it. I don't want to go too crazy with a server generating the licenses code and verifying them and everything, basically I want to include the licenses somewhere on the app and ask the user for licenses on first use then verify it locally. Of course it would easily be hackable but I think it is the best solution for my situation.

So what method would you recommend? Also it would be helpful if there is a guide about something like this, I couldn't really find much. Please do go into details if possible and thanks a lot for your help