rollback question

Greetings,

I have installed Postgresql 7.1.3 with --enable-locale and use it via PHP 4.0.6. My problem is that no kind of rollback works. The following piece of code

logon

insert into sorrend values ("proba");
rollback;

logoff

does not rollback anything but inserts the record into the table. The same applies when one of the queries from a series fail. Everything up to the failed query is committed, nothing is rolled back. I assumed that everything would be rolled back to the last savepoint. I thought that everything between logon and logoff is one transaction. Any ideas why this happens? Am I wrong at one of my assumptions?

Best Regards,

Hegyvari Krisztian