rollback()
$dbh->rollback()Use this function to undo a transaction that has not yet
been committed. This can only be used with transactional (e.g., InnoDB
or BDB) tables. It requires that the database handle was created with
the AutoCommit attribute set to false or 0, and
that the changes were not committed using the
commit() function or by any other method that
might unintentionally commit a transaction.