PHP always receives empty POST with XmlHttpRequest

Question

I'm trying to do a XMLHttpRequest for a login via POST and have actually done this before. I tried to recycle my code from a working project and changed it slightly but it's not working anymore and I honestly have no clue why.

The sent POST is always empty.

        var user = document.getElementById("username").value;
    var pw = document.getElementById("password").value;

    if(user != "" && pw != "") {

        if (window.XMLHttpRequest) {
            // code for IE7+, Firefox, Chrome, Opera, Safari
            xmlhttp = new XMLHttpRequest();
        } else {
            // code for IE6, IE5
            xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
        }
        xmlhttp.onreadystatechange = function() {
            if (this.readyState == 4 && this.status === 200) {
                var response = this.responseText;
                console.log(response);
                if(response == "-1") {
                  //do stuff
                } else if(response == "1") {
                    window.location.href = "/dashboard/"
                }
            }
        };
        var params = "username="+user+"&password="+pw+"&do=login";
        xmlhttp.open("POST","provider/SessionServices.php",true);
        xmlhttp.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
        xmlhttp.send(params);

It actually does the request and I receive my error code for missing POST params. I also debugged using print_r($_POST) and had to see that there is never POST data sent.

EDIT: SessionServices.php

print_r($_POST);
if(isset($_POST["username"]) && !empty($_POST["username"]) && 
isset($_POST["password"]) && !empty($_POST["password"])) {
if ($_POST["do"] == "login") {
    //login request by xmlhhtp
    $user = $_POST["username"];
    $pw = $_POST["password"];


    //do stuff
} else {
    echo "XH-102"; //unknown action
}
} else {
 echo "XH-101"; //user,pw,do empty or not set

it always outputs "Array ( ) XH-101"

Answer 1

Okay, seems that I have found the issue. XMLHttpRequests (or just using POST?) seemingly not work locally or atleast using XAMPP. When I uploaded it to my webserver it just worked.