code: public function beginTransaction($isolationLevel = null)
      {
          $this->open();
  
          if (($transaction = $this->getTransaction()) === null) {
              $transaction = $this->_transaction = new Transaction(['db' => $this]);
          }
          $transaction->begin($isolationLevel);
  
          return $transaction;
      }
docs: Starts a transaction.
  @param string|null $isolationLevel The isolation level to use for this transaction.
  See [[Transaction::begin()]] for details.
  @return Transaction the transaction initiated