Override method to take argument from a derived subclass

I ended up a couple of times with a solution where i have two hierarchies of classes. Where classes from the first (Greeters) hiearchy uses classes from the second (Users).

Heres a ex:

class User {}
class Admin extends User {
    function getSecretMessage() {
        return "secret";
    }
}

class Greeter {
    public function hello(User $a) {
        echo "hello!";
    }
}

class AdminGreeter extends Greeter {
    public function hello(Admin $a) {
        parent::hello($a);
        echo "in addition heres a secret of mine: " . $a->getSecretMessage();
    }
}

Here i have User's and Greeter's, In PHP i receive the error (strict)

"Declaration of AdminGreeter::hello must be compatible with Greeter::hello"

I would like the AdminGreeter::hello to simply "extend" the Greeter::hello with data from a more specialized class (Admin).

What alternatives do i have to build something similar i PHP??

I guess the main problem is that PHP does not support "method overloading" and thereby if i were to send in a User instance to the AdminGreeter it would break. But if i had "method overloading", the Greeter::hello would simply be called if passed in a User instance.

It might be overall bad design, since i end up with this issue, maybe someone can point me to a better design for this problem.

As i side note i seem to have the same issue when developing Objective-C