add_action( 'plugin_loaded', array( 'wpdocs_class_name', 'instance' ) );

class wpdocs_class_name {
     
    private function __construct() {
        self::init();
    }
     
    public static function instance() {
        static $instance = null;

        if ( is_null( $instance ) ) {
                $instance = new wpdocs_class_name;
        }

        return $instance
    }
     
    public function init() {
        wp_die( __( 'Hello World!', 'text-domain' ) );
    }
     
}