从数据库读数据怎么为空白

配置文件
<?php
return array(
'TMPL_L_DELIM'=>'<{', //左定界符
'TMPL_R_DELIM'=>'}>', //右定界符
'DB_TYPE'=>'mysql',
'DB_HOST'=>'localhost',
'DB_NAME'=>'thinkphp',
'DB_USER'=>'root',
'DB_PWD'=>'root',
'DB_PORT'=>'3306',
);
?>
Action文件：
<?php
class IndexAction extends Action {
public function index(){
$User = M('User');
$list = $User->select();
$this->assign('list',$list);
}
}
显示层：
<volist name="list" id="vo">
<{ $vo.username }>
</volist>
怎么访问的时候页面空白