PHP从父级访问Child的受保护值

Question

I have this code that i want to do method abstraction in parent and child will define the property

class SuperClass{
    static protected $message = "This is the parent";

    public static function showMessage(){
        echo self::$message."<br/>";
    }
}

class SubClass1 extends SuperClass {
    static protected $message = "This is the first child";

}

class SubClass2 extends SuperClass {
    static protected $message = "This is the second child";
}

SuperClass::showMessage();
SubClass1::showMessage();
SubClass2::showMessage();

I would expect to see

This is the parent
This is the first child
This is the second child

But what i got is

This is the parent
This is the parent
This is the parent