Yii transaction where to start it - php

Question

I wonder whether it matters where to start the transaction.
Example 1:
$transaction = Yii::app()->db->beginTransaction();
try
{
$savedSuccessfully = $object->save();
$transaction->commit();
}
catch (Exception $ex)
{
$transaction->rollBack();
$result = $e->getMessage();
}
Example 2:
$transaction = Yii::app()->db->beginTransaction();
try
{
$object = $model()::model()->findByPk(1); //!!!!!!! - line
// what makes the difference
$savedSuccessfully = $object->save();
$transaction->commit();
}
catch (Exception $ex)
{
$transaction->rollBack();
$result = $e->getMessage();
}
Should transaction be started before selecting data from db or or just before updating/inserting data? Will yii take care of it instead of me?
Thanks

Answer 1

Example 2 would be the solution of choice.
By retrieving the model within the transaction, you make sure that it is consistent throughout your changes.
If you retrieve the model, like in example 1, outside the transaction, other threads/users could change the corresponding database entry before you commit your changes. So you could end up with potentially inconsistent data.

Answer 2

Actually 2nd one is correct , if you are saving data which is more critical like banking transaction or payment system then example 2 is very correct way. for example , you are doing some code like this.
insert into table 1
select from table 1
insert into table 2
update table 2
select from table 1.
so if you start transaction from first , it will rollback all query if any query fails which will be more efficient. for example in online payment system.