using AJAX to check username duplication. there's another submit button on form, should not use submit whole page to check username availability.

in template:
        <DIV id='usercheck'>&nbsp;</DIV>
        <?php
 
        echo submit_to_remote('ajax_submit', 'Check Username', array(
            'update'   => 'usercheck',
            'url'      => '/user/checkusername',
        ));

in action class, check dupliation:

    public function executeCheckusername(){
        $username=$this->getRequestParameter('obj_username');
        $c = new Criteria();
        $c->add(UserPeer::USERNAME, $username);
        $user = UserPeer::doSelectOne($c);
 
        if ($user) {        // username already exists
            $this->setFlash('username_exists', 'y');
            return sfView::SUCCESS;
        } else {
            $this->setFlash('username_exists', 'n');
            return sfView::SUCCESS;
        }
 
    }

in checkusernameSuccess, display error message:

    <?php
    if($sf_flash->get('username_exists') == 'y'){ ?>
    <FONT COLOR="RED">Username Not Available</FONT>
    <?php } else { ?>
    <FONT COLOR="GREEN">Username Available</FONT>
    <?php } ?>