Help with QUERY_STRING

[twinklerip]

i got this example somewhere

<?phpini_set("register_globals", "on");if($_SERVER['QUERY_STRING'] == "") {    // Main Page    echo "main page!!";} elseif($_SERVER['QUERY_STRING'] == "about") {    // About page    echo "about page!!";} elseif($_SERVER['QUERY_STRING'] == "contact") {    // Contact page    echo "contact page!!";} else {    // Page not found    echo "Oops.. Page not found (404)";}?>

when i use http://localhost/Untitled-2.phpit says

Notice: Undefined index: QUERY_STRING in c:\Inetpub\wwwroot\Untitled-2.php on line 12main page!!

how do i get rigd of the notice?and one more thing what is ini_set("register_globals", "on") for?