在PHP中试图将数组Foreach转换为XML

我有一个数组，我希望foreach将键和值显示为XML。页面正在提出<$value>，仅此而已？有什么想法吗？谢谢！

$XML_a = array ("Ticket_ID" => "12343456");
    $query = "";
 $string_top = 
"<varcor_api>
    <response>";
       foreach ($XML_a as $key => $value) {
          $query .= "<" . $key . "><" . $value . "></" . $key . ">";
       }
$string_bottom = "
    </response>
</varcor_api>
";
    echo $string_top . $query . $string_bottom;