It's quite possible that the book, if it's an older copy, might be using register_globals, which is evil, old and deprecated. Whatever you do, don't ever turn register_globals on. It will ruin your life and demand the soul of your first-born.
Register_globals makes all GET, POST, COOKIE fields available in your script as regular variables. While this seemed handy at first, it turns out it's a major security hole, which eventually lead to thousands of sites being defaced/ hacked.
As ParkinT shows, you need to specifically retrieve the variable you need from the superglobals it's in (GET, POST, COOKIE, ENV etc).
Just noticed you're using an online version of the guide. You might need to find an updated version of the guide, as this is a very very old version.