how to create this trigger?

I need to create a trigger that increment a value in a column
of table1 where a row is inserted in table 2

I have tried whit this but it just does nothing.

CREATE FUNCTION increment_value () RETURNS opaque AS
'DECLARE
code int4;
BEGIN
code := new.code;
UPDATE table1
SET value = value + 1
WHERE id = code;
RETURN NEW;
END;' LANGUAGE 'plpgsql';

CREATE TRIGGER insert_on_table2 BEFORE INSERT ON table2
FOR EACH ROW EXECUTE PROCEDURE increment_value();

Can you help me?