<input name="custom" type="hidden" id="custom" value="<?php echo"$user";?>">

$user=strip_tags($_GET['custom'])
$item_number=strip_tags($_GET['item_number'])
mysql_query("INSERT INTO paypal SET id='', User='$user', item_number='$item_number'");
mysql_query("UPDATE items SET item1='1' where User='$user'");