when does php stop php executing when user clicks stop

If database operations are underway when a user accesses a web page, then user clicks stop on the browser, does the php stop immediately too? I'd rather the php continue behind the scenes to fullfill all the actions I need it do to whether the user wants to see if happen or not. Overall I wonder how modphp keeps the code executing even if the browser drops the connection via stop. Any insight is greatly appreciated. thanks, -eric wood