Ajax for unique usernames

I always enjoy the logical thinking required for programming. For Symfony it is really nice how it all just flows together. I wanted to implement an ajax check for unique usernames. Something similar like how you see it on Twitter. Lets get started:
First create a validator in lib/validators/sfUniqueUserValidator.class.php

<?php
class sfUniqueUserValidator extends sfValidator
{
public function execute (&$value, &$error)
{
//check if the username exists
$c = new Criteria();
$c->add(sfGuardUserPeer::USERNAME, $value);
$user = sfGuardUserPeer::doSelect($c);
if (!empty($user))
{
$error = $this->getParameter('user_error');
return false;
}
return true;
}
public function initialize ($context, $parameters = null)
{
// Initialize parent
parent::initialize($context);
// Set default parameters value
$this->setParameter('user_error', 'This username is taken');
// Set parameters
$this->getParameterHolder()->add($parameters);
return true;
}
}

Then in your view template use:

<?php echo observe_field('rusername', array(
      'update'   => 'userstatus',
      'url'      => 'sfGuardAuth/checkuser',
      'with' => "'id='+$('rusername').value",
  )) ?>

this will monitor an input field called rusername, and submit its value to the sfGuardAuth/checkuser internal url.
And to glue it all together, in the actions:

  public function executeCheckuser()
  {
     $username = $this->getRequestParameter('id');
     $userValidator = new sfUniqueUserValidator();
     $userValidator->initialize($this->getContext());
     $error='none';
     if (!$userValidator->execute($username,$error))
     return $this->renderText($username.' is taken');
     return $this->renderText($username.' is available');
  }

Enjoy!
Ps. any tips for posting code in wordpress would be greatly appreciated, for me it does the strangest types of things.