Website coding question...

<?
// Redirects user to main site, with parameter "s" (state) as "tx" (Texas)
header("Location: http://www.mydomain.com/?s=tx");
?>

<?
// If the "s" (state) parameter isn't set, show generic message
if (!isset($_GET['s'])) {
     echo "Insert generic welcome message."
} else if ($_GET['s'] == "tx") {
     echo "Welcome, Texan..."
}
?>

<?
$states = array(
          "tx" => "Texan",
          "ak" => "Alaskan"
          );

// If the "s" (state) parameter isn't set or the state code doesn't exist in the array (user tampered), show generic message
if ((!isset($_GET['s'])) || (!array_key_exists($_GET['s'], $states))) {
     echo "Insert generic welcome message."
} else {
     $key = $_GET['s'];
     echo "Welcome, " . $states[$key] . "." // Shows "Welcome, Texan." etc.
}
?>

<?
$ref = $_SERVER['HTTP_REFERER'];
if ($ref == "the_referer_you_want") include ("file-to-include.php");
?>

<?
$ref = $_SERVER['HTTP_REFERER'];
if ($ref == "the_referer_you_want")
     include("file-to-include.php");
else
     include("file-to-include.php?correct_refer=no");
?>

<?
if (!isset($_GET['correct_refer']))
     echo "This is shown to people with your preferred referer.";
else 
     echo "This is shown to people without your preferred referer.";

if ($_GET['correct_refer'] == "no")
     echo "This is only shown to people without your preferred refer.";
?>