PDO Transaction

Hi,

I have a trouble with PDO transactions.

I would like to start using transactions, but I am not familiar with it. I
have got a 'There is no active transaction' exception, however, I am using
beginTransaction method, and also I have set PDO::ATTR_AUTOCOMMIT to false
right after connecting to the database.
In my whole code I have used the commit method only once.

Between beginTransaction and commit methods, as far as I know, I did not
use anything that could activate auto-commit. In my test code, there is
only an exec method of PDO, and an execute of PDOStatement. Maybe one of
these activated auto-commit?

And what are the possible commands which are able to activate auto-commit?
I know that the commit method can do that, but - as I already wrote - I
have issued it only once, and it is in the destructor of a singleton class.

So what could be the problem?