php - Someone has dropped a table in my database. How?

In my php scripts ALL input are "filtered" with mysqli_real_escape_string in this way:

$categoryid = mysqli_real_escape_string($link, $_GET['id']); 

$query = "SELECT categories.id AS cid, categories.title AS ctitle
              FROM categories
              WHERE cid=".$categoryid.";";


$rows = mysqli_query($link, $query);
$row = mysqli_fetch_array($rows);

Someone could tell me, how did he hacked my database and dropped the "category_post" table ?

register_globals is disabled