framework/db/Transaction.php
public function begin($isolationLevel = null)
    {
        if ($this->db === null) {
            throw new InvalidConfigException('Transaction::db must be set.');
        }
        $this->db->open();

        if ($this->_level === 0) {
            if ($isolationLevel !== null) {
                $this->db->getSchema()->setTransactionIsolationLevel($isolationLevel);
            }
            Yii::debug('Begin transaction' . ($isolationLevel ? ' with isolation level ' . $isolationLevel : ''), __METHOD__);

            $this->db->trigger(Connection::EVENT_BEGIN_TRANSACTION);
            $this->db->pdo->beginTransaction();
            $this->_level = 1;

            return;
        }

        $schema = $this->db->getSchema();
        if ($schema->supportsSavepoint()) {
            Yii::debug('Set savepoint ' . $this->_level, __METHOD__);
            $schema->createSavepoint('LEVEL' . $this->_level);
        } else {
            Yii::info('Transaction not started: nested transaction not supported', __METHOD__);
            throw new NotSupportedException('Transaction not started: nested transaction not supported.');
        }
        $this->_level++;
    }
public function begin($isolationLevel = null)
    {
        if ($this->db === null) {
            throw new InvalidConfigException('Transaction::db must be set.');
        }
        $this->db->open();

        if ($this->_level === 0) {
            if ($isolationLevel !== null) {
                $this->db->getSchema()->setTransactionIsolationLevel($isolationLevel);
            }
            Yii::debug('Begin transaction' . ($isolationLevel ? ' with isolation level ' . $isolationLevel : ''), __METHOD__);

            $this->db->trigger(Connection::EVENT_BEGIN_TRANSACTION);
            $this->db->pdo->beginTransaction();
            $this->_level = 1;

            return;
        }

        $schema = $this->db->getSchema();
        if ($schema->supportsSavepoint()) {
            Yii::debug('Set savepoint ' . $this->_level, __METHOD__);
            $schema->createSavepoint('LEVEL' . $this->_level);
        } else {
            Yii::info('Transaction not started: nested transaction not supported', __METHOD__);
            throw new NotSupportedException('Transaction not started: nested transaction not supported.');
        }
        $this->_level++;
    }
Begins a transaction.
@param string|null $isolationLevel The [isolation level][] to use for this transaction.
This can be one of [[READ_UNCOMMITTED]], [[READ_COMMITTED]], [[REPEATABLE_READ]] and [[SERIALIZABLE]] but
also a string containing DBMS specific syntax to be used after `SET TRANSACTION ISOLATION LEVEL`.
If not specified (`null`) the isolation level will not be set explicitly and the DBMS default will be used.

> Note: This setting does not work for PostgreSQL, where setting the isolation level before the transaction
has no effect. You have to call [[setIsolationLevel()]] in this case after the transaction has started.

> Note: Some DBMS allow setting of the isolation level only for the whole connection so subsequent transactions
may get the same isolation level even if you did not specify any. When using this feature
you may need to set the isolation level for all transactions explicitly to avoid conflicting settings.
At the time of this writing affected DBMS are MSSQL and SQLite.

[isolation level]: http://en.wikipedia.org/wiki/Isolation_%28database_systems%29#Isolation_levels

Starting from version 2.0.16, this method throws exception when beginning nested transaction and underlying DBMS
does not support savepoints.
@throws InvalidConfigException if [[db]] is `null`
@throws NotSupportedException if the DBMS does not support nested transactions
@throws Exception if DB connection fails