ajax用response.Text接受返回值是空

var XHR=new XMLHttpRequest();

    var account=document.getElementById("account");

    var password=document.getElementById("password");

    if(XHR==null)

        alert("creat a new object failed");

    else

        XHR.open("post","index.php",true);

    XHR.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');

        XHR.send();



        alert(XHR.responseText);

echo "hello";

?>