How do API's work? How do I make my website 'open source', allowing users to add modules or 'apps' to it?

Question

anonymous · Answer

you need to define interfaces so that they would be the standard structure of any class that need to be developed..
then the plugin developer will use your interface to develop the real extension...

then you need to load all the extensions from a specific folder on start up of your application.

anonymous · Answer

You write your code and separate it into two pieces: private and public code, other developers must have access to your public functions, so they would write apps. You can write any function that you want to make public and give access to it.

Open source, that means, that you put all your application code in free access to any person, and any person can modify it by himself.