Chat Code

<html>
<head>
<script>
    function loadXMLDoc(){
        var xmlhttp = new XMLHttpRequest();
        xmlhttp.onreadystatechange=function() {
            if (xmlhttp.readyState==4 && xmlhttp.status==200) {
                document.getElementById("chatFenster").innerHTML=xmlhttp.responseText;
            }
        }
        xmlhttp.open("GET","refresh.php",true);
        xmlhttp.send();
    }
    setInterval(loadXMLDoc, 1500);

    function clearText() {
        document.getElementById('text').value = '';
    }
</script>
</head>
<body onload="loadXMLDoc();" style="text-align: center;">
<?php
include("helper.php");
$database = connect();
if (isset ($_SESSION['userID'])){
    $user = getUser($database, $_SESSION['userID']);
}
if (isset($user)){
    if ($user != NULL){
    if (isset ($_POST['klasse'])){
        $_SESSION['klasse'] = $_POST['klasse'];
        }
        ?>
        <h2>Chat</h2>
        <div id="chatFenster" style="overflow:auto; height:700px; width:500px;text-align: left; margin-left: auto; margin-right: auto;">

        </div>
        <br>
        <br>
        <form name="senden" action="senden.php" method="post">
        <textarea name="text" id="text" style='height:30px; width:200px;'></textarea>
        <input type="submit" name="senden" value="senden" style='height:30px; width:70px; position: relative; top: -7px;'>
        </form>
        <form action="upload.php" enctype="multipart/form-data" method="post">
        <input type="file" name="file">
        <input type="submit" name="upload">
        </form>
        <button onclick="loadXMLDoc();">Refresh</button>
        <button onclick="clearText()">clear</button>
        <?php
    } else {
        echo "Du hast nicht die Befugnis!";
        header("refresh:5;index.php");
    }
} else {
    echo "Du hast nicht die Befugnis!";
    header("refresh:5;index.php");
}
?>
</body>
</html>