//application/controllers/AuthController.php
<?php
class AuthController extends Zend_Controller_Action
{
	public function loginAction(){
		$db = $this->_getParam('db');

		$loginForm = new Default_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 = $auth->authenticate($adapter);

			if($result->isValid()) {
				$this->_helper->FlashMessenger('Successful Login');
				$this->redirect('/');
				return;
			}
		}

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

//application/forms/Auth/Login.php
<?php
class Default_Form_Auth_Login extends Zend_Form
{
	public function init(){
		$this->setMethod('post');
		$this->addElement(
			'text', 'username', array(
				'label' => 'Username:',
				'required' => true,
				'filters' => array('StringTrim'),
			)
		);
		$this->addElement(
			'password', 'password', array(
				'label' => 'Password:',
				'required' => true,
			)
		);
		$this->addElement(
			'submit', 'submit', array(
				'ignore' => true,
				'label' => 'Login',
			)
		);
	}
}