I have read http://www.yiiframework.com/wiki/24/creating-a-dependent-dropdown/ (Dependen Dropdown Yii1), but I can't implement it in Yii2 because Yii2 does not have built-in AJAX functionality hem.. I searched about it and came across this post
Using a version control system, like Git, is nice. However, when building an extension from scratch and loading it via Composer, it adds a lot of pain in the butt steps. You have to commit your changes, update composer to pull them over, then notice there is an error, fix, commit, update. repeat.. I don't want all my baby steps under Git. Sure, I could edit my commit history, but c'mon. Just let me code! I will put it under version control once I get a baseline created. This guide will show you how to setup your composer.json to load a local directory (everything inside, live). You make a change, save the file, refresh the page, boom.