Site works on Windows with XAMPP but doesnt work on raspberry pi

Question

I created a website on Windows using PHP, MySQL and one Line of JS, and tested it with XAMPP. Works perfectly. If I load this page on my PI now, it doesn't work anymore. The php script stop working. PHP is up-to-date. I can't update MariaDB because the latest available version for the PI is 10.3.23-MariaDB-0+deb10u1.

Both times I get the error message :

PHP Notice:  Undefined index: SERVER_NAME in /var/www/test/inc/functions.php on line 5
PHP Fatal error:  Uncaught mysqli_sql_exception: No index used in query/prepared statement (null) in /var/www/test/inc/functions.php:497
Stack trace:
#0 /var/www/test/inc/functions.php(497): mysqli_stmt_execute()
#1 /var/www/test/opener.php(8): checkCurrentTimer()
#2 {main}
  thrown in /var/www/test/inc/functions.php on line 497

using mysqli_report(MYSQLI_REPORT_OFF); this error is no longer displayed.

What is my error? The databases on the PI and my PC are identical (exported from the PC, imported into the PI and checked).

            <?php
//Looks like the next line causes the problem
            if (checkCurrentTimer(0)){
               echo '<div class="col">
                    <form method="post" action="'.$_SERVER['PHP_SELF'].'" style="margin-left:auto;margin-right:auto;display:flex;">
                        <input type="number" name="number" placeholder="Sec" style="max-width:60px"><p> </p>
                        <button type="submit" class="btn btn-block btn-dark" value="Zeitschaltung" name="Zeitschaltung" style="font-size:18px;">Zeitschaltung</button>
                    </form>
                </div>  ';
            }else{
                if(strtolower(checkCurrentTimer(1)) == strtolower($_SESSION['session_user'])){
                   ?>
                    <script src="/inc/js/jquery-3.5.1.js"></script>
                    <script type="text/javascript">
                    <!-- setTimeout("document.location.reload();",1000); -->
                   var initialTime = <?php echo timeleft($_SESSION['session_user']);?>;
                    var seconds = initialTime - 1;
                    function timer() {
                        var remainingSeconds = seconds;
                        if (remainingSeconds < 10) {
                            remainingSeconds = "0" + remainingSeconds;
                        }
                        document.getElementById('timer').innerHTML = "Dein Timer läuft noch <br>" +remainingSeconds+ "<br>Sekunden";
                        if (seconds == 0) {
                            clearInterval(countdownTimer);
                            document.getElementById('timer').innerHTML = "Completed";
                        } else {
                            seconds--;
                        }
                    }
                    var countdownTimer = setInterval('timer()', 1000);
                  </script>

                            <div class="col">
                                <form id="timer" class="btn btn-block btn-dark" name="return" style="color:green;font-size:14px;width:200px;margin-left:auto;margin-right:auto">
                                    Dein Timer läuft noch <br> <?php echo timeleft($_SESSION['session_user']);?><br> Sekunden
                                </form>
                            </div>

                   <?php
                }else{
                    echo '
                        <div class="col">
                            <form class="btn btn-block btn-dark" name="return" style="color:orange;font-size:14px;width:200px;margin-left:auto;margin-right:auto"">
                                Ein Timer läuft aktuell
                            </form>
                        </div>
                       ';
                }
            }
            ?>

The function:

function checkCurrentTimer($arg){
    include 'db.php';
    switch($arg){
        case 0:
            $sql = "SELECT COUNT(*) AS counts FROM timer;";
            $stmt = mysqli_stmt_init($connection);
            if (!mysqli_stmt_prepare($stmt, $sql)){
                // Leitet auf die Einstellungsseite weiter und meldet das ein Fehler aufgetreten ist
                mysqli_stmt_close($stmt);
                mysqli_close($connection);
                header("Location: ../module/oneklick.php?error=sqlerror&m=54");
                exit();
            }else{
                mysqli_stmt_execute($stmt);
                $result = mysqli_stmt_get_result($stmt);
                $row = mysqli_fetch_assoc($result);
                $counts = (int)$row['counts'];
                if($counts == 0 ){
                    mysqli_stmt_close($stmt);
                    mysqli_close($connection);
                    return true;
                }elseif($counts > 0 ){
                    mysqli_stmt_close($stmt);
                    mysqli_close($connection);
                    return false;
                }

            }
            break;
        case 1:
            $sql = "SELECT user FROM timer";
            $stmt = mysqli_stmt_init($connection);
            if (!mysqli_stmt_prepare($stmt, $sql)){
                // Leitet auf die Einstellungsseite weiter und meldet das ein Fehler aufgetreten ist
                mysqli_stmt_close($stmt);
                mysqli_close($connection);
                header("Location: ../opener.php?error=sqlerror&m=55");
                exit();
            }else{
                mysqli_stmt_execute($stmt);
                $result = mysqli_stmt_get_result($stmt);
                $row = mysqli_fetch_assoc($result);
                $user = $row['user'];
                if(empty($row['user'])){
                    $user = "%Empty%" ;
                }
                mysqli_stmt_close($stmt);
                mysqli_close($connection);
                return $user;
            }
            break;
        default:
            header("Location: ../opener.php?error=sqlerror&m=55");
            exit();
    }
}

Pi phpmyadmin: https://prnt.sc/uq5rgw

XAMPP phpmyadmin: https://prnt.sc/uq5rur

DB table timer structure: https://prnt.sc/uq5scx