You use gii to create a CURD suite of code and got the error of

Unknown Class – yii\base\UnknownClassException

Unable to find ‘backend\controllers\xxxxxController’ in file: /opt/php-5.4.37/html/backend/controllers/xxxxxController.php. Namespace missing?

the reason is, you shall not use ‘app\controllers’ as namespace, but e.g. in this case, backend\controllers