$config = new Zend_Config_Ini(APPLICATION_PATH. '/configs/application.ini', 'development');

        $db = new Zend_Db_Adapter_Pdo_MySql(array(
        'adapter'=>$config->resources->db->adapter,
        'host'=>$config->resources->db->params->host,
        'username'=>$config->resources->db->params->username,
        'password'=>$config->resources->db->params->password,
        'dbname'=> $config->resources->db->params->dbname));

        $loginForm = new Application_Form_Auth_Login();

        if($loginForm->isValid($_POST))
        {
            $adapter = new Zend_Auth_Adapter_DbTable(
                $db,
                'users',
                'username',
                'password',
                'MD5(CONCAT(?, password_salt))'
            );

            $adapter->setIdentity($loginForm->getValue('username'));
            $adapter->setCredential($loginForm->getValue('password'));

            $auth = Zend_Auth::getInstance();

            $result = $adapter->authenticate(); // <---- error

            if($result->isValid())
            {
                $this->_helper->FlashMessenger('Login Successful!');
                $this->_helper->redirector('index');
                return;
            }

        }

        $this->view->loginForm = $loginForm;