An example of how to use Yii2 as a micro-framework for primarily its ActiveRecord ORM. You can use it with or without its router.
My purpose was to make it as simple as possible (pretty much using ActiveRecord as a standalone feature), therefore the example is defaulting to not using Yii's router. Then you don't need to add folders, index.php, controllers, configure pretty URLs, etc. If you on the other hand want to use the router add calling the run() method in bootstrap.php ((new yii\web\Application($config))->run();), enable controllerNamespace in the config array, and add the necessary files and folders as described in Using Yii as a Micro-framework.
Download or clone this repo and run composer install.
To use a database set your database name, user name and password in the bootstrap.php, and add your model/ActiveRecord classes in there as well. If you rather want to add a folder for models and create a single file for each of your models you can of course do so. Again, this sample aims to be just for a very small project, so we wanted to keep it as simple as possible. Can always easily reorganize if the need would ever occur - as long as we keep everything nicely separated in classes.
Now you can access mypage.php through whatever web server you use, and add pages as you please. Just require bootstrap.php in all your pages.