PHP call_user_func

<?php
function my_call_back_function(){
    echo "hello world!";
}
class MyClass{
    static function myCallBackMethod(){
        echo "Hello World!";
    }
}
call_user_func('my_call_back_function');
call_user_func(array("MyClass","myCallBackMethod"));
$obj=new MyClass();
call_user_func(array($obj,"myCallBackMethod"));
call_user_func('MyClass::myCallBackMethod');
class A{
    public static function who(){
        echo "A\n";
    }
}
class B extends A{
    public static function who(){
        echo "B\n";
    }
}
call_user_func(array("B","parent::who"));