if (isset($_GET['page']))
{
    // The GET argument is present. Lets include the page.
    include($_GET['page'] . ".php");
}
else
{
    // The GET argument is not present. Lets give the poor guy some links!
    echo('<p><a href="index.php?page=1">Page 1</a></p>');
    echo('<p><a href="index.php?page=2">Page 2</a></p>');
    echo('<p><a href="index.php?page=3">Page 3</a></p>');
}