Ajax文本文件不起作用，但PHP

I have an ajax setup like this:

var $ = function(id) {return document.getElementById(id);};

var ajax = {
    send: function(url, callback, method, data, async) {
        var x = new XMLHttpRequest();

        x.open(method, url, async);

        x.onreadystatechange = function() {
            if (x.readyState == 4) {
                callback(x.responseText, x.status);
            }
        };

        if (method == "POST") {
            x.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
        }

        x.send(data);
    },
    query: function(data) {
        var query = [];

        for (var key in data) {
            query.push(encodeURIComponent(key) + "=" + encodeURIComponent(data[key]));
        }

        return query.join("&");
    },
    get: function(url, data, callback, async) {
        ajax.send(url + "?" + ajax.query(data), callback, "GET", null, async);
    },
    post: function(url, data, callback, async) {
        ajax.send(url, callback, "POST", ajax.query(data), async);
    }
}

and this is my function that I call to recieve the contents of a txt file:

function update(text) {
    var d = $("display");

    ajax.get("chat.txt", {}, function(t) {
        d.innerHTML = t;

        d.scrollTop = d.scrollHeight;
    }, true);
}

but this won't work and always show an old text that was in the txt an hour ago.

So I have found another solution:

function update(text) {
    var d = $("display");

    ajax.get("display.php", {}, function(t) {
        d.innerHTML = t;

        d.scrollTop = d.scrollHeight;
    }, true);
}

, to call a PHP file which code is:

<?php
    echo file_get_contents("chat.txt");
?>

Why can't I just do a call to the txt???

It have worked before in other php projects.

I use WAMP Server 2.

Considering you're using WAMP on localhost, give your /www/ directory full permissions by right clicking on it and clicking on Properties then Security.