Like how do you make a plugin like buying a rank on servers and it registers in game? Like when you buy something on a store externally you get it in game. Eg: buy VIP on a store then you get this chat prefix in game [VIP]Indexfire How would i do that?
Call online APIs on those websites when something happens in-game? This is more related to web development, not plugin development. What you need to know in plugin development is Utils::getURL() and the most basic use of AsyncTask.
Use BuyCraft, it works fine. Until you have a solid understanding of PHP and the PocketMine API you should start coding something a little easier though - it sounds like you want to run before you can walk!
I want to code minigame plugins for servers, not mess around with my friends on a private server- which is why im so rushed :/
This is something that takes even more time and experience, and this is something that you cannot rush. You can't expect to be able to compose a great symphony in a few months when the bare knowledge you have to music is to sing a few simple tunes.
Elaborate. MySQL is just one of the many tools you can use to achieve this. You could as well not touch MySQL at all in your plugin, and just post HTTP requests to the external website that has an API for doing whatever stuff you want to do.