如何在 yii 中下载 xml 文件

我想在yii中创建和下载xml文件，所以我有编写代码来创建xml文件，它工作正常，但是下载xml文件
时出错，我已经尝试了代码

//xml structure
$xmldata = '<?xml version="1.0" encoding="utf-8"?>';
$xmldata .= '<MemberBill>';
foreach ($model as $model)
{
    $xmldata .= '<Bill>';
    $xmldata .= '<BillNo>'.$model->bill_no.'</BillNo>';
    $xmldata .= '</Bill>';
}
$xmldata .= '</MemberBill>';
if(file_put_contents('memberBill.xml',$xmldata)) // this code is working fine xml get created
{
    //echo "file created";exit;
    header('Content-type: text/xml');   // i am getting error on this line
    //Cannot modify header information - headers already sent by (output started at D:'xampp'htdocs'yii'framework'web'CController.php:793)
    header('Content-Disposition: Attachment; filename="memberBill.xml"');
    // File to download
    readfile('memberBill.xml');        // i am not able to download the same file
}