Session variables (how do I pass session data to a function)

I have a question on how to do something with Postgres that I think
of by the name of "Session Variables".

What I want to do is build a table that will be filled in by a trigger,
it will contains three fields in addition to the fields in the table
that causes the trigger that will contain the time, user and the "proxy
user" for the change (this is an audit table) I have the following example
that handles the first two extra fields correctly:

--------------------------------------------------------

CREATE TABLE partypersons (
partyid TEXT,
firstname TEXT,
middlename TEXT,
lastname TEXT,
personaltitle TEXT,
suffex TEXT,
employmentstatus TEXT,
residencestatus TEXT,
maritalstatus TEXT,
PRIMARY KEY (partyid)
)
;

CREATE TABLE partypersons_audit (
audit_when TIMESTAMP,
audit_who TEXT,
partyid TEXT,
firstname TEXT,
middlename TEXT,
lastname TEXT,
personaltitle TEXT,
suffex TEXT,
employmentstatus TEXT,
residencestatus TEXT,
maritalstatus TEXT,
PRIMARY KEY (partyid,audit_when)
)
;

ALTER TABLE partypersons ADD
FOREIGN KEY ( partyid ) REFERENCES partys ( partyid )
;

-- AUDIT FOR PARTYPERSON

CREATE OR REPLACE FUNCTION fauditpartypersons() RETURNS TRIGGER AS $fauditpartypersons$
DECLARE
BEGIN
IF (TG_OP = 'UPDATE') THEN
IF (OLD.firstname = NEW.firstname) THEN
NEW.firstname := NULL;
END IF;
IF (OLD.middlename = NEW.middlename) THEN
NEW.middlename := NULL;
END IF;
IF (OLD.lastname = NEW.lastname) THEN
NEW.lastname := NULL;
END IF;
IF (OLD.personaltitle = NEW.personaltitle) THEN
NEW.personaltitle := NULL;
END IF;
IF (OLD.suffex = NEW.suffex) THEN
NEW.suffex := NULL;
END IF;
IF (OLD.employmentstatus = NEW.employmentstatus) THEN
NEW.employmentstatus := NULL;
END IF;
IF (OLD.residencestatus = NEW.residencestatus) THEN
NEW.residencestatus := NULL;
END IF;
IF (OLD.maritalstatus = NEW.maritalstatus) THEN
NEW.maritalstatus := NULL;
END IF;
END IF;
INSERT INTO partypersons_audit SELECT now(), user, NEW.*;
RETURN NULL;

END;
$fauditpartypersons$ LANGUAGE plpgsql;

CREATE TRIGGER auditpartyperson AFTER UPDATE OR INSERT
ON partypersons FOR ROW
EXECUTE PROCEDURE fauditpartypersons ()
;

--------------------------------------------------------

As you can see the primary key and the time and user are always set
while the other fields are NULL unless an update (or insert) changes
them.

What I want to do is to add to this another field that will be NULL
UNLESS the session sets a value to the "user" for that the middle-wear
system is acting for.

What this will be used for will be a web system that will access the
DB as the user apache but I want to do two things:

1) log to the audit table all changes even if they are done outside of
the web interface (this is done by the above system)

2) in the same log allow the system (apache) to also say who it is "acting
for"

Thanks.

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