PHP Fatal error: Call to a member function getUser() on a non-object-Collection of common programming errors

I am new to OOP and trying to recall a function from a parent class in Facebook WP plugin, I get the error from the file of the parent class

my code is:

class check_fb extends Wdfb_Model {
function __construct() {
            Wdfb_Model::get_wp_user_from_fb();
    }
}

//REST OF THE CODE

here is the code for the class http://codepad.org/d7MtD9i7 knowing that it is a class within a WP facebook plugin and works well within that plugin

generally what I am trying to do is define the property $this->fb->api('/me'); in the my class to get the user FB information, since I can’t declare the class FACEBOOK which is already declared from the plugin.