a nice simpler alternative to $GET and $POST is $_REQUEST as it grabs the variable you desire from any user method.
But as stated above, more than likely you are calling a variable that is not set, something also tells me you are running this script from your machine because most scripts will execute regardless of if the variable is set or not, because most hosts disable the fatal/parse error on that little nip. And as a suggestion to you, being new to PHP and all, I would suggest working with versions 4.3.X before moving on to PHP 5.X.X seeing as it was only recently released either alittle over a year ago or almost a year ago, you might come across bugs in that version that might throw you off the learning experience which is very important in landing a good foundation and understanding of the syntax