Calling methods from others classes

Hi, I have this piece of code that I have created: class userQueries { public function numberUsers() { $get_users=mysql_query("SELECT * FROM users WHERE online < NOW()") or exit("../includes/error.php"); } } class usersOnline extends userQueries { public function usersOnline() { echo mysql_num_rows($this->numberUsers); } } How do I request specific methods in other classes? The error that comes up is: mysql_num_rows(): supplied argument is not a valid MySQL result resource which is somewhat expected as $this only refers to the variables and there are no variables called numberUsers. Cheers!