add_menu_page not available from Class

Hello everyone!

I created a Help-Class for Administration Menus, where I wanted to encapsulate all nessecary names and functionality required by wordpress to add one.

The class structure looks like this:
file AdminMenu.php

abstract Class AdminMenu {
  // ... various variables
  // __construct() {}
  // ... various final methods

  abstract function register_with_wp();
}

Class ToplevelAdminMenu extends AdminMenu {
  // __construct() {}
  function register_with_wp() {
    add_menu_page (....);
  }
}

file Pluginname.php

require_once(DIRECTORY.'AdminMenu.php'); // doesnot create fatal errors
class Pluginname {
  __construct () {
    $this->adminmenu = new ToplevelAdminMenu(); // works fine
    $this->adminmenu->register_with_wp(); // breaks because of undefined function within this function called 'add_menu_page'
    add_menu_page(...); // PHP knows function
  }
}

In my Sub-Class I’ve got a problem: it says, I cannot access an undefined function. But: I create these objects within a plugin-file, so it has to be defined – or nor?

Where can I access these functions like add_menu_page, cause, as it is described in the PHP-Docs, required files should have the same variables-visibility as the line where it was required.

I’m still working on this but am thankful for any help provided.

If it’s an PHP-specific problem, I am sorry for posting it in this forum, but I think that the wordpress communitiy is much more aware of the structure of wordpress than any other PHP-programmer.