null values to be replaced by a default value

Hi,
   To return one value not null, i've created a function :

CREATE FUNCTION isnull (int4,int4 ) RETURNS int4 AS '       BEGIN               IF ($1 IS NULL) THEN
RETURN$2;                ELSE                RETURN $1;               END IF;       END;       ' LANGUAGE 'plpgsql';
 

select isnull(my_int,0) as number, sample_id from my_table;

but I got
number  |sample_id
------+---------             |       13             |       15

What should I do to have '0' in the column 'number' instead of NULL ?
Thanks,
   nuch