Accessing function parameters within double quotes

Hi

No matter what value of parameter I call the function below with, the value of $2 is always 2 within the addition operation. Is $2 inaccessible within the double quotes? If so how do I amend the function to get it to work?

CREATE FUNCTION get_data(DATE, INTEGER) RETURNS SETOF data AS '
DECLARE
row data%ROWTYPE;
BEGIN
FOR row IN
SELECT * FROM data
WHERE startdate >= $1
AND startdate <= $1 + interval ''$2 months''
LOOP
RETURN NEXT row;
END LOOP;
END;
' LANGUAGE 'plpgsql';

Regards

John Duffy