xmlhttprequest responsetext为null

I'm trying to make basic HTML Server connection, therfore I want to call and JS function which should call an PHP file just schoing "hello world". Everything is working so far but the response I get seems to be null. I've read through various tutorials but I did not find an answer, hope someone can help me.

            var xhttp = new XMLHttpRequest();
        xhttp.onreadystatechange = function() {
            if (xhttp.readyState == XMLHttpRequest.DONE && xhttp.status==200 && xhttp.responseText!=null) {
                alert("Responestext: " + xhttp.responseText + "  ENDE");
            }else if(xhttp.status==403){
                alert("forbidden");
            }else if(xhttp.status==404){
                alert("not found");
            }else if(xhttp.responseText==null) {
                alert("response text = null");
            }else{
                alert("Error");
            }
        };
        xhttp.open("GET", "http://URL/fahrgemeinschaft/login.php", true);
        xhttp.send(null);

I expect the output to be "Responsetext: hello world ENDE" but all I get is "Error". I get two alert boxes saying "Error" as well.

The problem is your onreadystatechange handler is called for every ready state change event, not just when it is done.

You should skip the events that are not done:

var xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function() {
    if (xhttp.readyState != XMLHttpRequest.DONE) {
        return;
    }
    if (xhttp.status==200 && xhttp.responseText!=null) {
        alert("Responestext: " + xhttp.responseText + "  ENDE");
    }else if(xhttp.status==403){
        alert("forbidden");
    }else if(xhttp.status==404){
        alert("not found");
    }else if(xhttp.responseText==null) {
        alert("response text = null");
    }else{
        alert("Error");
    }
};
xhttp.open("GET", "http://messenger.hopto.org:8080/fahrgemeinschaft/login.php", true);
xhttp.send(null);

Or to make it easier on yourself, so long as you don't need to support obsolete browsers, just use the onload event.

var xhttp = new XMLHttpRequest();
xhttp.onload = function() {
    if (xhttp.status==200 && xhttp.responseText!=null) {
        alert("Responestext: " + xhttp.responseText + "  ENDE");
    }else if(xhttp.status==403){
        alert("forbidden");
    }else if(xhttp.status==404){
        alert("not found");
    }else if(xhttp.responseText==null) {
        alert("response text = null");
    }else{
        alert("Error");
    }
};
xhttp.open("GET", "http://messenger.hopto.org:8080/fahrgemeinschaft/login.php", true);
xhttp.send(null);