// ----- form -------------

<?php

class Application_Form_Login extends Zend_Form {

    public function init() {

        $this->setMethod('post');

        $this->addElement('text', 'username', array(
            'label' => 'Username: ',
            'required' => TRUE,
            'attribs' => array(
                'size' => '20'
            ),
            'validators' => array(
                new Zend_Validate_Alnum(),
                new Zend_Validate_StringLength(array('min' => 4, 'max' => 30))
            ),
            'filters' => array('StripTags', 'StripNewLines')
        ));

        $this->addElement('password', 'password', array(
            'label' => 'Password: ',
            'required' => TRUE,
            'attribs' => array(
                'size' => '20'
            ),
            'validators' => array(
                new Zend_Validate_StringLength(array('min' => 8))
            ),
            'filters' => array('StripTags', 'StripNewLines')
        ));

        $this->addElement('submit', 'submit');
    }

}

// ------------ view -------------------
<h2>User Login</h2>
<p>To login to your account please enter your username and password below...</p>
<?php if($this->loginMessage) echo '<p>'.$this->loginMessage.'</p>'; ?>
<?php echo $this->form; ?>

//------------- controller --------------
public function loginAction() {
        // action body
        $userForm = new Application_Form_Login();
        $this->view->form = $userForm;
        if ($this->_request->isPost() && $userForm->isValid($_POST)) {

            $data = $userForm->getValues();

            $bootstrap = $this->getInvokeArg('bootstrap');
            $db = $bootstrap->getResource('db');
            if ($db instanceof Zend_Db_Adapter_Abstract) {
                $db->getProfiler()->setEnabled(true);
            }

            $auth = new Zend_Auth_Adapter_DbTable($db);
            $auth->setTableName('users')
                    ->setIdentityColumn('username')
                    ->setCredentialColumn('password')
                    ->setIdentity($data['username'])
                    ->setCredential($data['password']);
            $result = $auth->authenticate();
            if ($result->isValid()) {
                $userForm->populate($data);
            } else {
                $this->view->loginMessage = "Sorry, your username or password was incorrect: " . $data['username'] . "/" . $data['password'];
            }
        }
    }