namespace ...

use Symfony\Component\HttpKernel\Event\GetResponseEvent;
use Symfony\Component\HttpKernel\HttpKernel;
use Symfony\Component\HttpFoundation\Session\Session;
use Symfony\Bridge\Monolog\Logger;

class RequestListener
{

    public $session;
    public $logger;

    public function __construct( Session $session, Logger $logger ) {
        $this->session = $session;
        $this->logger  = $logger;
        //leave blank
    }

    public function onKernelRequest( GetResponseEvent $event ) {

        if ( HttpKernel::MASTER_REQUEST != $event->getRequestType() ) {
            // don't do anything if it's not the master request
            return;
        }
        else {

            $session      = $this->session;
            $logger       = $this->logger;
            $current_time = time();

            //if they're logged in and it hasn't been more than X seconds since their last request
            if ( $session->get( 'auth' ) ) {
                $last_seen = (int) $session->get( 'last_seen' );
                if ( $last_seen > ( $current_time - 10 ) ) { //timeout: half hour
                    //regenerate session ID:
                    $session->migrate();
                    $logger->info( "Regenerating session ID, last_seen=" . $last_seen );
                }
                else {
                    //destroy session
                    $session->clear();
                    $session->migrate();
                    //$session->invalidate(); //shortcut for clear() & then migrate()

                    //set session variable to acknowledge timeout
                    $session->setFlash( 'error', 'You were automatically logged out due to inactivity' ); 

                    $logger->info( "Session timeout (upon request from returning user)" );

                }
                $session->set( 'last_seen', $current_time );
            }
            else {
                $session->set( 'last_seen', $current_time );
            }
        }
    }
}