AJAX没有回应

So I'm trying to just alert out my "Hello" from my php file, it's pretty straight forward but it's not working for some reason.

    function dynamic()
{
    if(window.XMLHttpRequest)
    {
        xmlhttp = new XMLHttpRequest();
    }
    else
    {
        xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    }

    xmlhttp.onreadystatechange = function()
    {
        if(xmlhttp.readyState == 4 && xmlhttp.status == 200)
        {   
            window.alert(xmlhttp.responseText);
        }
    }

    xmlhttp.open("POST", "hello.php", true);
    xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
}

dynamic();

PHP

<?php 
echo "Hello";
?>

HTML

<html>
<head>
    <title>Hello</title>
    <script src="dynamic.js"></script>
</head>
<body>

test

</body>
</html>

I can't get the alert box to alert my "Hello". It should just come straight from responseText.

You didn't invoke send method. It sends the request ^MDN

xmlhttp.send();

If you want to put any data in POST put it in send methods parameter as urlencoded form.

xmlhttp.send("foo=bar&hello=world");

try this:

    function dynamic()
{
    if(window.XMLHttpRequest)
    {
        xmlhttp = new XMLHttpRequest();
    }
    else
    {
        xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    }

    xmlhttp.onreadystatechange = function()
    {
        if(xmlhttp.readyState == 4 && xmlhttp.status == 200)
        {   
            window.alert(xmlhttp.responseText);
        }
    }

    xmlhttp.open("GET", "hello.php", true);
    xmlhttp.send(null);
}

dynamic();

I think you forgot to send the request:

xmlhttp.send();

try this

xmlhttp.setRequestHeader("Content-type", "text/plain");

xmlhttp.setRequestHeader("Content-type", "text/html");

instead of

xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");

and you have to put

xmlhttp.send(null);

beside the answer, logging in php file is a useful behaviour in similar situations.