Paul Reinheimer recently discovered something interesting about how PHP handles users aborts (hitting the stop button in the browser, closing it) and the steps the language follows after that.
I was under the impression that when the user hit stop, the script stopped. Which is: bad, and wrong. PHP doesn't detect that the user has terminated the connection, it has no clue, it obligingly continues processing along, until it attempts to send information to the user.
The "ignore_user_abort" configuration option in the php.ini controls some of this, allowing for you to specify whether or not the script should go ahead and finish out even after PHP has discovered that the client has been disconnected. There's also a function that allows you to specify the same thing on a script-by-script basis (in both PHP4 and PHP5).