如何在laravel 4.2中创建用于发送SMS的XML对象？

I am trying to send sms to mobile numbers of users in my project. I need to send sms by using xml format given by sms provider and it is shown below.

<MESSAGE>     
    <AUTHKEY>Authentication Key </AUTHKEY>     
    <SENDER>SenderID</SENDER>     
    <ROUTE>Template</ROUTE>     
    <CAMPAIGN>XML API</CAMPAIGN>     
    <SMS TEXT="message1" >         
        <ADDRESS TO="number1"></ADDRESS>         
        <ADDRESS TO="number2"></ADDRESS>     
    </SMS>
</MESSAGE>

My program forsending SMS is given below

public function sendUserSms()
    {
        $usernos = array();
        $users = User::all();
        foreach ($users as $key => $user) {
            $xmlpno = new SimpleXMLElement("<ADDRESS></ADDRESS>");
            $xmlpno->addAttribute('TO', $user->mobile);
            $usernos[] = (string) $xmlpno;
        }
    }
$xmlmsg = '<MESSAGE>     
                    <AUTHKEY>Authentication Key </AUTHKEY>     
                    <SENDER>SenderID</SENDER>     
                    <ROUTE>Template</ROUTE>     
                    <CAMPAIGN>XML API</CAMPAIGN>     
                    <SMS TEXT="hello_test_message" >'.implode($usernos).'    
                    </SMS>
                </MESSAGE>';
var_dump($xmlmsg);

It shows nothing. Anybody know what is the problem? Can anyone help?