下载.doc无法正常工作，但在同一浏览器上打开，为什么

Question

下载.doc无效但在同一浏览器上打开

if ($stmt->execute(array($userID))) 
{
$stmt->execute();
header("Content-type: application/vnd.ms-word");
header('Content-type: text/plain charset=utf-8');
header('Content-Disposition: attachment; filename=export.doc');

while ($row = $stmt->fetch()){
$username=htmlspecialchars($row['userName']);
<table width='90%' height='60%' align='center' border='1' dir='rtl'>

download .doc  not working but open on a same  browser why