Yaf_Session::has

<?php
class BaseController extends Yaf_Controller_Abstract
{
    public function requireLogin()
    {
        $session = Yaf_Session::getInstance();
        $session->start();

        if (!$session->has("user_id")) {
            $this->redirect("/account/login");
            return false;
        }

        return true;
    }
}
?>