src/Db/Connection.php
public function beginTransaction($isolationLevel = null)
    {
        $beginCmd = $this->createCommand("BEGIN")->execute();
        $isolationCmd = isset($isolationLevel)
            ? $this->setTransactionIsolationLevel($isolationLevel)
            : resolve(true);
        $promises = [$beginCmd, $isolationCmd];
        return new LazyPromise(function() use ($promises) {
            return allInOrder($promises);
        });
    }
public function beginTransaction($isolationLevel = null)
    {
        $beginCmd = $this->createCommand("BEGIN")->execute();
        $isolationCmd = isset($isolationLevel)
            ? $this->setTransactionIsolationLevel($isolationLevel)
            : resolve(true);
        $promises = [$beginCmd, $isolationCmd];
        return new LazyPromise(function() use ($promises) {
            return allInOrder($promises);
        });
    }
Begin transaction
@param string|null $isolationLevel
@return LazyPromiseInterface