Tanax is correct, just because an class 'extends' another it doesn't mean that the 'base' class will have any knowledge of any derived objects. To add this functionality you would need to store the object in an array in the base object and use a getter to access it (as Tanax suggested you could use the magic get interceptor function __get)

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mysql> SELECT * FROM `users` WHERE `users`.`clue` > 0;
Empty set (0.00 sec)