Multi-row update w. plpgsql function

Given a set of checkbox values that are submitted through an html form,
how do you loop through the submitted values to update more than one row
in a table?

Imagine a table called 'message_table':

mid | message | status
----+---------+-------
  1  |  Text1   |  H
  2  |  Text2   |  H
  3  |  Text3   |  H
  4  |  Text4   |  H

A web page presents the user with all messages flagged with 'H'. User
checks messages 1,3 and 4 and submits form.
(i.e. approved=1&approved=3&approved=4)

After performing postgreSQL update, rows 1, 3 and 4 would be updated to:

mid | message | status
----+---------+-------
  1  |  Text1   |  A
  2  |  Text2   |  H
  3  |  Text3   |  A
  4  |  Text4   |  A

I have never written a plpgsql function, but tried:

CREATE OR REPLACE FUNCTION update_messages(approved integer) RETURNS
integer AS
$body$
DECLARE
 new_status varchar;
 new_sample record;

BEGIN
 new_status := 'A';

 FOR new_sample IN SELECT * FROM message_table WHERE status='H' ORDER BY
mid LOOP
  UPDATE message_table SET status = new_status
  WHERE mid = approved;
 END LOOP;

 RETURN 1;
END;
$body$
LANGUAGE 'plpgsql' VOLATILE CALLED ON NULL INPUT SECURITY INVOKER;

I call the function with:
SELECT update_messages(<xsp-request:get-parameter name="approved"/>);

I'm using apache cocoon, which is why you see the variable placeholder:
<xsp-request:get-parameter name="approved"/>);

Unfortunately, the function only updates the first value submitted (mid
1), and doesn't loop through the other two values submitted.

Can someone help this novice from getting ulcers?

Thanks for your help!

Daniel