I tryed the following:

<?php

session_name('user');
session_start();
// sets an 'user' cookie with session id.
// This session stores the user's login data.
// Its an ordinary session.
$USERSESSION=$_SESSION; // saving this session
$userSessId=session_id();
session_write_close(); //closing this session

session_name('chatroom');
session_start();
// set a 'chatroom' cookie but contains the same session id and points to the same data :( .
// This session would be used to store the chat text. This session would be
// shared between all users joined in this chat room by setting the same session id for all members.
// by calling session_regenerate_id would make a diffent session id for every user and it won't be a chat room anymore.

?>

So I want to do a chat like thing with sessions. On the client side it would be done with ajax that polls this php page in every 5-10 seconds. Sessions may be cached in the server's memory so it can be accessible fast. I can store the chat in the database but my service runs on a free webhost which is limited, only 4 mysql connections allowed at a time which is almost nothing. I try to touch my database as least times as possible.