executing a script on browser exit?

Hello all! I've only recently picked up PHP, but I'm starting to gett the hang of it. My question is this: is there any way to execute a script when the user leaves the site or closes the window? My current design sets a cookie that tracks the number of pages per session that a user views. When the user exits the site, I would like to write that number to a file. I'm sure I could do the same thing by reading/writing to file every page, but that seems like quite the waste of system resources. Does anyone have any ideas or design alternatives? - Craig Simons