在调用超类中声明的函数时引用子类对象

How do I make a function always refer to it's current class object, even when called from a subclass?

The following code will echo a: a

I need to echo b: b

class superclass {

    private $a = 'a';

    public function getObjectVars() {
        return get_object_vars($this);
    }

}

class subclass extends superclass {

    private $b = 'b';

}

$obj = new subclass();

$vars = $obj->getObjectVars();
foreach ($vars as $name => $value) {

    echo $name . ': ' . $value;

}

I think you need to make the vars in subclass "protected" instead of private in order for the parent methods to access them..