Documentation comments in yii2 say us that $this pointer in layout files refer to yii\web\View, but I'm confused that how this pointer can used outside of his class!?
If you debug the code deep enough, you will see this https://github.com/yiisoft/yii2/blob/master/framework/base/View.php#L341
They require a file in the method, that is why $this point to class :)