Trigger function

Hello,
I'm trying to write a trigger that upon inserting a value in a table
another table is decremented. Something that could be used in an
inventory system, for example. I know it can be done easily outside
POSTGRES with PHP but in this case I would like the database to handle
it.

Example:

create table order (
order_number integer not null default
nextval('order_number_seq'::text),
itemid number integer references inventory,
item_amt integer,
cost numeric(3,2),
constraint order_pk primary key(order_number)
);

create table inventory (
itemid integer not null default nexval('itemid_seq'::text),
onhand integer not null default 0,
onorder integer not null default 0,
desc varchar(30) not null,
cost numeric(3,2) not null,
constraint inventory_pk primary key(itemid)
);

Can you give me some direction on how to do this. In this case decrement
the inventory by the amount of the order for that particular item. I'm
not sure how to pass variables to triggers and/or functions.

Thank you

--
Kent L. Nasveschuk <kent(at)wareham(dot)k12(dot)ma(dot)us>