Scripts tend to die, and that's not usually nice. We do not want to show the user a fatal error nor a blank page (display errors off) . PHP has a function called register_shutdown_function which lets us set up a function which is called at execution shutdown. What this means is that our function will be executed when our script is done executing / dying and PHP execution is about to shut down.
He suggests various things that can be done with the functionality, including checking for successful script execution (via a false variable that can be checked for success).