QF registerRule with methods

Hello, a little problem with a 'callback' rule using registerRule. For database access we had written a class with methods wich worked very well. Also registering them as rules worked fine. Quick form uses the syntax calling methods directly in the following way Class::method(). Now we have the case where we need to inherit one class using some methods from the base class as callback methods. But using the below mentioned syntax we obtain a fatal error "Call to a member function on a non-object in ...". # Usual way using methods as callback rule: $form->registerRule('rule_name', 'callback', 'method', 'class'); $form->addRule('formfield', 'Message', 'rule_name', 'some_param'); # is similar to: class::method('formfield_value', 'some_param'); For making inheritance work, we need a call to a method of an instance of our class wich extends the base class. Testing something like the following: # This works without any error. # Inheritance works. Return values true/false work. $obj = new class(); $obj->method('formfield_value', 'some_param'); Now the questions. Does anyone know how to use instances in registering rules? How is the right syntax? I browsed the documentation several times trying something like: $obj = new class(); $form->registerRule('rule_name', 'callback', $obj->method ); or $form->registerRule('rule_name', 'callback', $obj->method, $obj ); or $form->registerRule('rule_name', 'callback', $obj->method, 'class' ); or $form->registerRule('rule_name', 'callback', $obj->method, 'file_to_class' ); but none of them works. Thank you Thomas