Extend a class plugin

This works for me:

class father {
    var $user;
    function __construct() {
        add_action( 'init', array( &$this, 'set_user' ) );
    }
    function set_user() {
        $this->user = wp_get_current_user();
    }
}

class child extends father {
    function __construct() {
        parent::__construct();
    }
    function user_id(){
        return $this->user->ID;
    }
}

$father = new father();
$child = new child();

add_action( 'admin_notices', 'test_stuff' );
function test_stuff() {
    global $child;
    print '<pre>Child: ' . print_r( $child->user_id(), true ) . '</pre>';
}