httpRequest.onreadystatechange = function()     {

        if (httpRequest.readyState === XMLHttpRequest.DONE) {

                console.log("Request is done.");

                if (httpRequest.status === 200) {
                    // perfect!

                    console.log("Response from server: 200 OK (Bingo!)");

                    if (httpRequest.responseText == "submitted") {
                        document.getElementById("vent").innerHTML = "Good luck chuck!";
                    }

                    console.log("ResponseText: '" + httpRequest.responseText + "'");


                } else {

                    console.log("There was a problem with the request. The response code was: " + httpRequest.status);
                }
        }
    }

    sendMessage = document.getElementById('button');

    // sendMessage.onclick = function() {

    function mySubmitFunction() {

        document.getElementById("myform").style.display="none";

        document.getElementById("ventcontainer").style.display="block";

    myName = document.getElementById("myName").value;
    var encodedMyName = encodeURIComponent(myName);

    myEmail = document.getElementById("myEmail").value;
    var encodedMyEmail = encodeURIComponent(myEmail);

    myPhone = document.getElementById("myPhone").value;
    var encodedMyPhone = encodeURIComponent(myPhone);

    myWork = document.getElementById("myWork").value;
    var encodedMyWork = encodeURIComponent(myWork);


    httpRequest.open("POST", "theSeparatePHPfile", true);

    // THIS IS NEEDED TO SEND THE DATA AS URL ENCODED VARIABLES
    httpRequest.setRequestHeader("Content-type", "application/x-www-form-urlencoded");


    httpRequest.send("&myName="+encodedMyName+"&myEmail="+encodedMyEmail+"&myPhone="+encodedMyPhone+"&myWork="+encodedMyWork);

    }

$servername = "XXX";
$username = "XXX";
$password = "XXX";
$dbname = "XXX";


// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);
$conn->set_charset("utf8");

// Check connection
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}

$name =  htmlentities($_POST[myName]);
$email = htmlentities($_POST[myEmail]);
$phone = htmlentities($_POST[myPhone]);
$work = htmlentities($_POST[myWork]);


//(MESSAGE, USER)
$sql = "INSERT INTO Skjema_fraBruker (Name, Email, Phone, Work)
    VALUES ('$name', '$email', '$phone', '$work')";

if ($conn->query($sql) === TRUE) {
    echo "submitted";
} else {
    echo "Error: " . $sql . "<br>" . $conn->error;
}

<form id="myform" name="theform" class="group" onsubmit="event.preventDefault();mySubmitFunction();">
<fieldset id="XXX" title="XXX">
    <legend>XXX</legend>
    <!-- <img id="XXX" src="XXX"> -->
    <h1 id="headertext">XXX</h1>
    <h2 id="senternavn">XXX</h2>
    <span id="formerror" class="error"></span>
    <ol>
        <li>
            <label for="myname">Name</label>
            <input type="text" name="Name" id="myName" title="XXX" autofocus required placeholder="NAME HERE" >
        </li>
        <li>
            <label for="myemail">Email</label>
            <input type="email" name="Email" required id="myEmail" autocomplete="off" autofocus required placeholder="XXX@XXX.com" >
        </li>
        <li>
            <label for="mytelephone">Phone:</label>
            <input type="tel" name="Phone" id="myPhone"  placeholder="xx-xxx-xx" >
        </li>
        <li>
            <label for="mywork">Where do you work</label>
            <select name="mywork" id="myWork">
                <option>Choose...</option>
                <option value="XXX">XXX</option>
                <option value="XXX">XXX</option>
            </select>
            <input type="submit" id="button">
            <!--<button id="button">Send</button>-->
        </li>
    </ol>
</fieldset>