trigger/stored procedure error

Hello,

I never worked with SP/triggers in MySQL, I have this problem. On
webpage I'd like to show online users, so idea is very simple: in one
select (SP) make all necesary things. I begin with:

CREATE TRIGGER online_insert
AFTER UPDATE ON subs_online
FOR EACH ROW
DELETE FROM subs_online WHERE ts < (unix_timestamp() - 180);
--------------
CREATE PROCEDURE online_insert(ip char(15))
INSERT INTO subs_online
(ip, ts) VALUES (ip, unix_timestamp())
ON DUPLICATE KEY UPDATE ts=unix_timestamp();

IN PHP: $this->db->DbQuery("call
online_insert('{$_SERVER['REMOTE_ADDR']}')");

But I get error:
query: call online_insert('84.47.19.168')
error: Can't update table 'subs_online' in stored function/trigger
because it is already used by statement which invoked this stored
function/trigger.

I tried change "AFTER UPDATE" to something else, but nothing worked as
expected.

Could someone help me on this issue please ?

Thanks a lot.