PHP Editor Review - Postgres Manual -> Plpgsql-trigger:

CREATE TABLE emp (
empname text,
salary integer,
last_date timestamp,
last_user text
);

CREATE FUNCTION emp_stamp () RETURNS TRIGGER AS '
BEGIN
-- Check that empname and salary are given
IF NEW.empname ISNULL THEN
RAISE EXCEPTION ''empname cannot be NULL value'';
END IF;
IF NEW.salary ISNULL THEN
RAISE EXCEPTION ''% cannot have NULL salary'', NEW.empname;
END IF;

-- Who works for us when she must pay for?
IF NEW.salary < 0 THEN
RAISE EXCEPTION ''% cannot have a negative salary'', NEW.empname;
END IF;

-- Remember who changed the payroll when
NEW.last_date := ''now'';
NEW.last_user := current_user;
RETURN NEW;
END;
' LANGUAGE 'plpgsql';

CREATE TRIGGER emp_stamp BEFORE INSERT OR UPDATE ON emp
FOR EACH ROW EXECUTE PROCEDURE emp_stamp();