- All framework classes are now namespaced, and the name prefix `C` is removed.

- The format of path alias is changed to `@yii/base/Component`.
  In 1.x, this would be `system.base.CComponent`. See guide for more details.

- The root alias `@yii` now represents the framework installation directory.
   In 1.x, this is named as `system`. We also removed `zii` root alias.


- `CList` is renamed to `Vector`, and `CMap` is renamed to `Dictionary`.
  Other collection classes are dropped in favor of SPL classes.

- `CFormModel` is removed. Please use `yii\base\Model` instead.