php - Yii2: why "use Yii" the yii is capitalized -

when use yii helper class, why using namespace statment "use yii;" ,the yii capitalized,while namespace definited "namespace yii;".

yii (as in use yii;) name of class , since yii 2 follows basic coding standard psr-1 (among others) starts capital letter.

namespace yii; defined class baseyii yii extends.

class yii not defined within namespace , why can use use yii;

notice yii in use yii; not same 1 in namespace yii; - first 1 class, second namespace.