I add this class to library/My/Validate/PasswordConfirmation.php

<?php 
require_once 'Zend/Validate/Abstract.php';
class My_Validate_PasswordConfirmation extends Zend_Validate_Abstract
{
    const NOT_MATCH = 'notMatch';

    protected $_messageTemplates = array(
        self::NOT_MATCH => 'Password confirmation does not match'
    );

    public function isValid($value, $context = null)
    {
        $value = (string) $value;
        $this->_setValue($value);

        if (is_array($context)) {
            if (isset($context['password'])
                && ($value == $context['password']))
            {
                return true;
            }
        } elseif (is_string($context) && ($value == $context)) {
            return true;
        }

        $this->_error(self::NOT_MATCH);
        return false;
    }
}
?>

then I create two field in my form like this :

       $userPassword = $this->createElement('password', 'user_password');
    $userPassword->setLabel('Password: ');
    $userPassword->setRequired('true');
    $this->addElement($userPassword);

    //create the form elements user_password repeat
    $userPasswordRepeat = $this->createElement('password', 'password_confirm');
    $userPasswordRepeat->setLabel('Password repeat: ');
    $userPasswordRepeat->setRequired('true');
    $userPasswordRepeat->addPrefixPath('My_Validate','My/Validate','validate');

    $userPasswordRepeat->addValidator('PasswordConfirmation');
    $this->addElement($userPasswordRepeat)

everything is good but when i submit form always I get the 'Password confirmation does not match' message ? What's Wrong in my code