Something

<?php
class My_Validate_PasswordConfirmation extends Zend_Validate_Abstract
{
    const NOT_MATCH = 'notMatch';

    protected $_mustEqual;

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

    public function setMustEqual($value)
    {
        $this->_mustEqual = (string) $value;
    }

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

        if ($value !== $value) {
            $this->_error(self::NOT_MATCH);
             return false;
        }

        return true;
    }
}