key( $GLOBALS[‘wp_registered_sidebars’] ) is always showing the same value

$GLOBALS[‘wp_registered_sidebars’] is a multidimensional array (an array of arrays) so you have to loop over this array and execute the key() function for each member

Now I think I know what is what you want to get. If you want to get the id of the widget’s parent sidebar, the widget area where the widget is assigned, you won’t be able to do through the global wp_registered_sidebars (maybe I’m wrong but using that global is not working for me both in frontend and admin area). You will have to use the arguments of the widget() method of the widget class:

add_action( 'widgets_init', 'cybnet_widgets_init' );
function curioso_widgets_init() {
     register_widget('cybnet_widget');
}


class cybnet_widget extends WP_Widget {
 function cybnet_widget() {
        $widget_ops = array( 'classname' => 'cybnet_widget', 'description' => ''  );
        $control_ops = array( 'width' =>  '' => '', 'id_base' => '' );
        $this->WP_Widget( 'cybnet_widget','cybnet_widget', $widget_ops, $control_ops );
}

function widget( $args, $instance ) {
            //Here is the id of the widget's parent sidebar
            $parent_sidebar = $args['id'];

    .....Rest of the widget code....