How to insert data in database using Ajax (without JQuery)?

I'm creating a shoutbox based on php, mysql and ajax for my website. Now I want the user to be able to write a message, open the saving file using Ajax and insert the message to the database with php. Everything is working fine - except for the ajax part. Somehow it doesn't open the saving file.

html:

<input type='text' id='shoutbox-comment' name='shoutbox-msg' style='width: 179px;' value='My message'>  
<input type='submit' name='shoutbox-submit' value='ok' onclick='shoutbox-send();'>

I'm using the framework Medoo for any database request. I've tried the code using a normal html form and the file itself is working fine.

php "shoutbox-write.php":

if (!empty($_POST['msg'])){
    $userid = $_SESSION['id'];

    $message = substr($_POST['msg'],0,200);
    $message = xss($message);

    $now = time();
    $database->insert("shoutbox", ["userid" => $userid, "message" => $message, "time" => $now]);
}

Now comes the problem part. I've tried various things, I even changed the whole script to GET to see if it's working. Nothing works. The first javascript code is one I took from here. The second one is the one I used before. Both are not working though. Please note that I want to solve this without using JQuery.

function shoutbox-send() {
    var xmlHttp = new XMLHttpRequest();
    var url="<?php echo PATH; ?>divers/shoutbox-write.php";
    var message = document.getElementById('shoutbox-comment').value;
    var parameters = "msg="+message;
    xmlHttp.open("POST", url, true);

    //Black magic paragraph
    xmlHttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    xmlHttp.setRequestHeader("Content-length", parameters.length);
    xmlHttp.setRequestHeader("Connection", "close");

    xmlHttp.onreadystatechange = function() {
        if(xmlHttp.readyState == 4 && xmlHttp.status == 200) {
        document.getElementById('shoutbox-comment').value="";
        }
    }
    xmlHttp.send(parameters);
}
function shoutbox-send() {
    var xhttp = new XMLHttpRequest();
    xhttp.onreadystatechange = function() {
        if (this.readyState == 4 && this.status == 200) {
            var message = document.getElementById('shoutbox-comment').value;
            document.getElementById('shoutbox-comment').value = "";
        }
    };
    xhttp.open("POST", "<?php echo PATH; ?>divers/shoutbox-write.php", true);
    xhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    xhttp.send("msg=" + message + "");
}

I'd appreciate your help.

Edit: Solved! Function names must not contain hyphens. It's working now