How to return a resultset/table from a sql function?

Hi,

Is it possible to return the following (parameterized) qyery from a sql or plpsql function, and if so, what is the
syntax?

SELECT{     (SELECT COUNT(klantnummer)         FROM abo_klt WHERE abonnement = $1),
        (SELECT SUM(aantal_abonnementen)     FROM abo_klt WHERE abonnement = $1));




Groeten,

Joost