why not do something which guarantees there'll be no collisions, as well as makes sure that no one can change a GET parameter to view something they shouldn't: using a salt, combine the id and its hash.
$salt = "salty";
$key = sha1($salt . $id) . "-" . $id;
// 0c9ab85f8f9670a5ef2ac76beae296f47427a60a-5
even if you do accidentally stumble upon two numbers which have the exact same sha1 hash (with your salt), then the $key will still be different and you'll avoid all collisions.
