Porblème formulaire authentification avec zend

[b]class IndexController extends Zend_Controller_Action[/b]
{
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   public function loginAction()
    {
	    require_once 'application\forms\auth\login.php';
		
        $db = $this->_getParam('db');
 
        $loginForm = new Default_Form_Auth_Login();
 
        if ($loginForm->isValid($_POST)) 
		{
 
           $adapter = new Zend_Auth_Adapter_DbTable(
                $db,
                'users',
                'username',
                'password'
                );
          
		    $adapter->setTableName('users');
            $adapter->setIdentity($loginForm->getValue('username'));
            $adapter->setCredential($loginForm->getValue('password'));
            
			
			 
                     
						
            $auth   = Zend_Auth::getInstance();
            $result = $auth->authenticate($adapter);
  
            if ($result->isValid()) 
			{
                $this->_helper->FlashMessenger('Successful Login');
                $this->_redirect('/');
                return;
            }
			else
			{
			  echo $loginForm->render($this->view);
			}
			
	
 
        }
		else
		{
		
		  echo $loginForm->render($this->view);
		  
		}
 
        $this->view->loginForm = $loginForm;
 
    }

$result = $auth->authenticate($adapter);

    protected function _authenticateQuerySelect(Zend_Db_Select $dbSelect)
    {
        try {
            if ($this->_zendDb->getFetchMode() != Zend_DB::FETCH_ASSOC) {
                $origDbFetchMode = $this->_zendDb->getFetchMode();
                $this->_zendDb->setFetchMode(Zend_DB::FETCH_ASSOC);
            }

            if (isset($origDbFetchMode)) {
                $this->_zendDb->setFetchMode($origDbFetchMode);
                unset($origDbFetchMode);
            }
        } catch (Exception $e) {
            /**
             * @see Zend_Auth_Adapter_Exception
             */
            require_once 'Zend/Auth/Adapter/Exception.php';
            throw new Zend_Auth_Adapter_Exception('The supplied parameters to Zend_Auth_Adapter_DbTable failed to '
                                                . 'produce a valid sql statement, please check table and column names '
                                                . 'for validity.', 0, $e);
        }
        return $resultIdentities;
    }

$auth   = Zend_Auth::getInstance();
$result = $auth->authenticate($adapter);

$result = $adapter->authenticate();

  public function _execute(array $params = null)
    {
       
	   
	   try {
            if ($params !== null) {
                return $this->_stmt->execute($params);
            } else {
                return $this->_stmt->execute();
            }
        } catch (PDOException $e) {
            require_once 'Zend/Db/Statement/Exception.php';
            throw new Zend_Db_Statement_Exception($e->getMessage(), (int) $e->getCode(), $e);
        }
		
    }