Stop process when user close window browser

Hi all, I am using PHP 4.3.2 and MYSQL database. I think this question has been asked many times, however not solutions. The problem is that I have a php script which connect to db and update data to db which took length of time to be completed. However sometimes user accidentally close the window browser or kill the window browser. The php which a server script language will not know that the window browser has been closed , so the program execution is keep running . This will choke the memory in the server and thus choke the web page. So ,are there ways to stop program from executing when user close the browser window. Thanks - weetat