Why does PEAR DB give "DB Error: extension not found [DB Error: extension not found].."

I'm moving a small set of PHP/Postgres applications from Ubuntu 9.x to 10.10. When I try to run the application, I get this error:

DB Error: extension not found [DB Error: extension not found] ** pgsql://jean@/appx

The code is:

            require_once('DB.php');
            $db=DB::connect("pgsql://jean@/appx");
            if (PEAR::isError($db)) {
                      die($db->getMessage()."  ".$db->getUserInfo()." ");
            }
            if (DB::isError($db)){
                        die($db->getMessage());
            }

When I installed DB, I noticed that it's been supersceded by MDB2, what do I have to change to make it work with MDB2, or what do I need to do make the DB code work?

Thanks.

Jean