PHP Web Auditing and Authorization

Dear all,

Imagine I have two users "Maria" and "Ana" using a PHP site.
There is a common Postgres user "phpuser" for both.
I'm creating audit tables to track the actions made by each PHP site user.

*I have used the following code:*

CREATE OR REPLACE FUNCTION MinUser_audit() RETURNS TRIGGER AS $usr_audit$
BEGIN
--
-- Create a row in MinUser_Audit to reflect the operation
performed on MinUser,
-- make use of the special variable TG_OP to work out the operation.
--
IF (TG_OP = 'DELETE') THEN
INSERT INTO MinUser_audit VALUES (DEFAULT, 'D', now(),
*user*, OLD.*);
RETURN OLD;
ELSIF (TG_OP = 'UPDATE') THEN
INSERT INTO MinUser_audit VALUES (DEFAULT, 'U', now(),
*user*, NEW.*);
RETURN NEW;
ELSIF (TG_OP = 'INSERT') THEN
INSERT INTO MinUser_audit VALUES (DEFAULT, 'I', now(),
*user*, NEW.*);
RETURN NEW;
END IF;
RETURN NULL; -- result is ignored since this is an AFTER trigger
END;
$usr_audit$ LANGUAGE plpgsql;

Everything seems to wok fine except the *use*r information I'm getting, in
this case "*phpuse*r".
I would like to have not the postgres user but the PHP site user (*Maria or
Ana*).

How can I pass the PHP site user (Maria or Ana) into Postgres in a clever
way?

I have done several web searches and found nothing for Postgres. I found a
solution for oracle:
http://www.oracle.com/technetwork/articles/dsl/php-web-auditing-171451.html
*
They use a "client identifier" feature.* Is there a similar way to do this
in Postgres?

Thanks in advance.
Gabriel