What happens when user hits the Stop button?

Hi all, I have a script that opens some files and does some generally messy stuff while executing, and then cleans up after itself right before terminating. It is very important that this script not be terminated half way through its processing, because then it would leave the system in a messy state. Thus, I need to make sure that even if the user hits the 'Stop' button in his/her browser (or the connection closes for some other reason), my script still executes until it is finished. Does anyone know how to do this? (I know that it is possible to do this, because I can remember reading about it in the PHP3 docs a while back. But I can't find the reference anymore. I believe it's a php.ini setting, but that's about all I can remember. Hopefully someone can point me in the right direction.) Thanks! -mike