framework/db/Connection.php
public function beginTransaction($isolationLevel = null)
    {
        $this->open();

        if (($transaction = $this->getTransaction()) === null) {
            $transaction = $this->_transaction = new Transaction(['db' => $this]);
        }
        $transaction->begin($isolationLevel);

        return $transaction;
    }
public function beginTransaction($isolationLevel = null)
    {
        $this->open();

        if (($transaction = $this->getTransaction()) === null) {
            $transaction = $this->_transaction = new Transaction(['db' => $this]);
        }
        $transaction->begin($isolationLevel);

        return $transaction;
    }
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