return new RedirectResponse(Drupal::url('<front>', [], ['absolute' => TRUE]));
  return new TrustedRedirectResponse('http://www.bbc.com/');

<?php

namespace Drupalachva_ssoController;

use DrupalCoreControllerControllerBase;


class SsoController extends ControllerBase{
    public function ssoLogout(){
        unset($_SESSION['name']);

        if(isset($_SESSION['name'])){
            $status = $_SESSION['name'] .' is logged in';
        }else{
            $status = 'is logged out';
        }
        return[
            '#type' => 'markup',
            '#markup' => '<p>' . $status . '</p>' ,
        ];


    }
}