(1)text/plain

       服务器端：

              header('Content-Type: text/plain');

              echo 'succ';

       客户端：

              if(xhr.responseText==='succ'){ ... }

  (2)text/html

       服务器端：

              header('Content-Type: text/html');

              echo "<tr><td>$data</td></tr>";

       客户端：

              tbody.innerHTML = xhr.responseText

  (3)application/javascript

       服务器端：

              header('Content-Type: application/javascript');

              echo "alert($data); f1(); f2($data)";

       客户端：

              eval( xhr.responseText )

  (4)application/xml

       服务器端：

              header('Content-Type: application/xml');

              echo "<bookList><book>$b</book></bookList>";

       客户端：

              var document = xhr.responseXML

  (5)application/json

       服务器端：

              header('Content-Type: application/json');

              //echo "[ {"bname":"","price":35.5},{} ]";

              $list = ...;

              echo json_encode($list);

       客户端：

              var obj = JSON.parse( xhr.responseText )