[Adrian Schneider]

$db->num_rows($resource) is a function, it returns the number of found rows from the given results ($resource).

PHP Code:

$result = $db->query_read("
    SELECT *
    FROM table
    WHERE rating > 3
    LIMIT 5
");

echo $db->num_rows($result); 


That should display an integer between 0 and 5.

Assuming your table has a userid field, do a query for that userid and if $db->num_rows() returns true then deny them access. As JumpD said, we can't really help much without more info.

Good luck.