This is the core framework code of Yii 2.
The preferred way to install the Yii framework is through composer.
Either run
composer global require "fxp/composer-asset-plugin:1.0.0-beta4"
composer require --prefer-dist "yiisoft/yii2 *"
or add
"yiisoft/yii2": "*",
to the require section of your composer.json.