AJAX with XMLHttpRequest doesn't send data

I want to build a simple program using XMLHttpRequest to calculate the area of the triangle. I used this code for client-side;

<body>
    <form>
        <label for="txtLength">Length</label>
        <input type="text" id="txtLength" name="txtLength"><br><br>
        <label for="txtWidth">Width</label>
        <input type="text" id="txtWidth" name="txtWidth"><br><br>
        <input type="hidden" name="submitted" value="1">
        <input type="button" name="Calculate" value="Calculate" onclick="calArea();">
    </form><br><br>

    <div id="showArea">Enter Values and click Calculate.</div>

    <script type="text/javascript">
        function calArea() {
            var len = document.getElementById("txtLength").value;
            var wid = document.getElementById("txtWidth").value;
            var sub = 1;

            var xhttp = new XMLHttpRequest();

            xhttp.onreadystatechange = function () {
                if (this.readyState == 4 && this.readyState == 200) {
                    document.getElementById("showArea").innerHTML = xhttp.responseText;
                }
            };

            xhttp.open("POST", "calculate_area.php", true);
            xhttp.send(len&wid&sub);
        }

</script>
</body>

This code is for the server side.

<?php 

print_r($_POST);
if (isset($_POST['sub'])) {
    $len = $_POST['len'];
    $wid = $_POST['wid'];
    $area = (($len*$wid)/2);
    echo $area;
}   
else{
    echo "Not input detected.";
}

?>

Even tried so many codes, It doesn't send the data to server side.

Solution

I found the mistake. I was sending the parameters as part of the URL, but need to send them as part of the request body.

Client-side code;

function calArea() {
    var len = document.getElementById("txtLength").value;
    var wid = document.getElementById("txtWidth").value;
    var sub = 1;

    var xhttp = new XMLHttpRequest();

    xhttp.onreadystatechange = function () {
        if (this.readyState == 4 && this.status == 200) {
            document.getElementById("showArea").innerHTML = xhttp.responseText;
        }
    };

    xhttp.open("POST", "calculate_area.php", true);
    xhttp.setRequestHeader("Content-Type", "application/json");
    xhttp.send(JSON.stringify({len: len, wid: wid, sub: sub}));
}

Server-side code;

if (isset($_POST['sub'])) {
    $len = $_POST['len'];
    $wid = $_POST['wid'];
    $area = (($len*$wid)/2);
    echo $area;
}   
else{
    echo "Not input detected.";
}