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
seriesfail. Everything up to the failed query is committed, nothing is rolled back. I assumed that everything would be
rolledback to the last savepoint. I thought that everything between logon and logoff is one transaction. Any ideas why
thishappens? Am I wrong at one of my assumptions? 

Best Regards,

Hegyvari Krisztian