php
\Yii::$app->getSession()->setFlash('error', 'This is the message');
\Yii::$app->getSession()->setFlash('success', 'This is the message');
\Yii::$app->getSession()->setFlash('info', 'This is the message');
Multiple messages could be set as follows:
php
\Yii::$app->getSession()->setFlash('error', ['Error 1', 'Error 2']);