Because Yii intends to be integrated nicely with third-party libraries, it does not define any global functions. Everything in Yii needs to be addressed with full class name or object scopes. For example, to access the current user, we need to use Yii::app()->user; to access application parameters, we need Yii::app()->params['name']; and so on. While editors like textmate can help alleviate the problem of these lengthy typings, it is worthwhile to define global shortcut functions to some commonly used method calls. They will make the application code look cleaner.
These features provide endless possibilities and unbelievable flexibility, but as current documentation does not give more than a few examples, it might be difficult to fully understand their internals and requirements.