$postData = "<sms>
<apikey>APIKEY</apikey>
<header>BAŞLIKheader>
<type>1</type>
<validity>---</validity>
<messages>
<gsm>";
foreach (explode(',', $_POST['numaralar']) as $numara) {
    $postData .= "<no>".$numara."</no>";
}
$postData .= "</gsm>
<msg>Mesaj İçeriği </msg>
</messages>
</sms>";