Notice: Undefined index:

i have this working fine on another site and have just replicated it but on this site it isnt working, can someone see what i am doing wrong?

i am having an  error come up on my success.php page when returning from a paypal payment..

Notice: Undefined index: OrderID in E:\Domains\b\website.com\user\htdocs\success.php on line 248

basically when the transaction is successful the return page is return.php, this page tells

<?php

session_start();

$colname_rsReturn = "-1";

if (isset($_SESSION['OrderID'])) {

  $colname_rsReturn = $_SESSION['OrderID'];

}

mysql_select_db($database_beau, $beau);

$query_rsReturn = sprintf("SELECT Fulfilled FROM beauSS13_orders WHERE beauSS13_orders.OrderID = %s", GetSQLValueString($colname_rsReturn, "int"));

$rsReturn = mysql_query($query_rsReturn, $beau) or die(mysql_error());

$row_rsReturn = mysql_fetch_assoc($rsReturn);

$totalRows_rsReturn = mysql_num_rows($rsReturn);

if ($row_rsReturn['Fulfilled'] == 1) {

  header ("Location: success.php");

} else header ("Location: failed-order.php");

?>

so the value of the order is 1 then it send to success.php to update the stock as follows

<?php

// *** Update the stock ***

$details_table = "beauSS13_orderdetails";

$ID_column = "OrderID";

$details_prodID = "ProductID";

$details_qty = "Quantity";

$XStock_TableName = "beauSS13_products";

$XStock_FieldName = "Stock";

$XStock_unID = "ProductID";

if (!session_id()) session_start();

if (isset($_SESSION["OrderID"])) {

  mysql_select_db($database_beau, $beau);

  $details_Source = "select * from " .  $details_table . " where " . $ID_column . " = " . $_SESSION["OrderID"];

  $detailsRS = mysql_query($details_Source, $beau) or die(mysql_error());

  $row_detailsRS = mysql_fetch_assoc($detailsRS);

  do {

    $XStock_qtySource = "select " . $XStock_FieldName . " from " .  $XStock_TableName . " where " . $XStock_unID . " = " . $row_detailsRS[$details_prodID] . "";

    $XStock_rsUpd = mysql_query($XStock_qtySource, $beau) or die(mysql_error());

    $row_XStock_rsUpd = mysql_fetch_assoc($XStock_rsUpd);

    if ($row_XStock_rsUpd[$XStock_FieldName] > 0) {

      $XStock_new = $row_XStock_rsUpd[$XStock_FieldName] - $row_detailsRS[$details_qty];

      if ($XStock_new < 0) $XStock_new = 0;

      $XStock_UpdSource = "update " . $XStock_TableName . " set " . $XStock_FieldName . " = " . $XStock_new . " where " . $XStock_unID . " = " . $row_detailsRS[$details_prodID] . "";

      $XStock_rsUpd = mysql_query($XStock_UpdSource, $beau) or die(mysql_error());

    }

  } while ($row_detailsRS = mysql_fetch_assoc($detailsRS));

  $XStock_rsUpd = null;

  $detailsRS = null;

  session_unregister("OrderID");

}

?>

but when i get to this page i am getting the error

Notice: Undefined index: OrderID in E:\Domains\b\website.com\user\htdocs\success.php on line 248

and the stock isnt getting updated

line 248 is <p>Order <?php echo $_SESSION['OrderID']; ?> Successful<br />

so this is telling me that the session isnt being carried accross?