[XAMPP] Call to a member function query()

Fatal error: Call to a member function query() on a non-object in C:\xampp\htdocs\live.php on line 39

linha 39 :  

$players_online_data = $SQL->query('SELECT * FROM players WHERE world_id = '.(int) $world_id.' AND cast > 0 AND online > 0 ORDER BY '.$orderby);