[Xenon]

this should be enough:

$users = $DB_site->query("SELECT COUNT(*) AS count, user.username, user.userid FROM user AS users
                   LEFT JOIN user ON (users.referrerid = user.userid)
                   WHERE users.referrerid <> 0 AND user.posts > 9
                   
                   GROUP BY users.referrerid
                   ORDER BY count DESC");