passing variables need help

Lo, I am a just starting with PHP and I am in the middle of building a site. What my question is; If I pass a variable to a page by www.mysite.com/index.php?page=support how do I grab that variable and use it in the index.php. I want to use the same index.php for the whole site and when I get the variable eg support pass it in to a variable like "pages/{$page}.php"; to stop any directory transversal attacks. I would appreciate it if anyone could tell me how to get the variable from the URL. Cheers, John F John.Flanagan@akad.ie