any way to check if a transaction is active?

I'm trying to find some method to detect wether a real transaction is in progress. With "real" I mean a transaction
explicitlystarted with "begin", not the 
auto-transactions postgres puts around single statements. Is there a function that can be used for this? It's ok for me
towrite a extension function in C if this is necessary, but I've no 
clue where to start?

The reason why I need this is this one:
* I use a database wrapper in PHP to stay compatible with Oracle
* whenever I issue a rollback without any prior transaction, the apache error log will have a line
NOTICE:  ROLLBACK: no transaction in progress
ROLLBACK

This makes the error log unreadable, so I'm trying to get rid of this. Any idea how?

Best regards,
    Mario Weilguni