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?