Need a simple PHP Plugin system/script to my PHP Smarty Template website

I have a website that's core php and based on the Smarty Template style. It has an MVC structure and every dev who's worked on it says it super easy to work with. I need to add a simple PHP Plugin system/script to it. Awhile back another developer added a Widget system, which might work if it's modified a little. Or we can add something new to handle the "plugins" feature. Basically I have a large set of functions that communicates with a famous API and I want to protect that code so I don't have to share it. That will allow me to share the rest of the website's source code sort of like a Website Template. I want users to be able to "install" (ie upload) a plugin and it will allow them to access certain features (ie functions) from my big functions file. That way I can charge for certain functions. Simple. I am guessing that I'll have a general "plug-in" connection file that communicates with my API functions like connecting to an API (ie with a header and authorization token, etc) - that will make the "connection". Then each plugin will simply "call" or reference a certain function or set of functions IF the user is a current subscriber. So we will simply check if their subscription status == 1. I already have a mobile app that connects to this API file of functions so I guess I just need the website to do something similar. Then I would just need to add the page to install/upload the plugins and the code and logic to connect the plugins to the various functions in my master API file.