HTML_QuickForm::addFormRule()

<?php
require_once ('HTML/QuickForm.php');

$form = new HTML_QuickForm();

// the function checks whether the passwords are the same
function cmpPass($fields)
{
    if (strlen($fields['passwd1']) && strlen($fields['passwd2']) &&
        $fields['passwd1'] != $fields['passwd2']) {
        return array('passwd1' => 'Passwords are not the same');
    }
    return true;
}

$form->addElement('password', 'passwd1', 'Enter password');
$form->addElement('password', 'passwd2', 'Confirm password');

$form->addFormRule('cmpPass');
?>