i need help with an undefined variable

I wrote a php script for a page that I hosted on a separate host and it worked perfectly. I recently installed php 4 onto my windows XP Pro build 2600 computer and it seems to work until I try to use varuables that look to the address bar for reference. I wanted to make it so that when somebody clicks "game" it would go to www.mywebsite.com/index.php?game=example and that would merely replace the body with the example page. It worked on my other host but when I tried it on my computer it says: Notice: Undefined variable: game in C:\Xitami\webpages\index.php on line 5 Notice: Undefined variable: game in C:\Xitami\webpages\index.php on line 8 My script says: <?php include("head.html"); if (!$game) { include("public.html"); } if ($game != NULL && $op != NULL) { include("http://mywebsite.com/example/" . "$game" . ".html"); } any ideas? please help. if there's any script i can type and have all the pages require it so that this can be defined that would work great.