yii\db\Migration::init PHP Method

    public function init()
    {
        parent::init();
        $this->db = Instance::ensure($this->db, Connection::className());
        $this->db->getSchema()->refresh();
        $this->db->enableSlaves = false;
    }

 /**
  * @inheritdoc
  */
 public function init()
 {
     parent::init();
     if (Yii::$app->db->driverName === 'mysql') {
         $this->tableOptions = 'ENGINE=InnoDB CHARACTER SET=utf8 COLLATE=utf8_unicode_ci';
     }
 }