who is online?

i have built a function to tell me how many users are on my site at any given time... <? //define function function bc_who ($who, $location) { //first we erase any expired entries //entries will expire after 5 mins $whoTime = time(); $whoTime = $whoTime - 300; mysql_query("Delete FROM bc_who where expire < $whoTime") or die(mysql_error()); //here we difine the variables needed to preform the check $whoExpire = time(); $whoIp = "$_SERVER[REMOTE_ADDR]"; //this will be changed as soon as user registration is finished $whoUser= "Guest"; //do the actual mysql queries mysql_query("Delete FROM bc_who where '$whoIp' = ip"); mysql_query("Insert INTO bc_who (id, user, ip, location, expire, status) VALUES (NULL, '$whoUser', '$whoIp', '$location', '$whoExpire', '$whoStatus')"); } //end who is function ?> this fuction works fine, however, i want to know if what i am about to ask is possible. the problem i have is, this function assumes that after five minutes if the user has not refreshed the page, or gone onto another page the user must be gone. in reality, i have pages users might be on for an hour or so without refreshing. i want my whos online to as acurate as possible so is there a way to do this on the fly? like, forgetting about the expire time and using a server side peice of code to communicate to the database, when there is no more communication the entry is deleted? please tell me there is a way, thank you in advance.