sessions using MySQL in xampp

Question

I have a problem with xampp when I want to start a session. I have created a user in a MySQL database. My signing form refers to this php script:

<?php
session_start();
?>
//...
<?php
$User = $_POST["User"];
$Pswd = $_POST["Pswd"];
//reference to current script $_SERVER['PHP_SELF'];
$con = open1($User, $Pswd);
if(isset($User)) {

    if (!empty($_POST['User']) && !empty($_POST['Pswd'])) {
        $username = mysqli_real_escape_string($con, $_POST['User']);    // get rid of special characters in 'User'
        $password = md5(mysqli_real_escape_string($con, $_POST['Pswd']));  // calculated md5 hash of the password string

        $checklogin = mysqli_query($con, "SELECT * FROM registrovany_uzivatel WHERE Uziv_jm = '" . $User . "' AND Heslo = '" . $Pswd . "'");

        if (mysqli_num_rows($checklogin) == 1) {
            //session_start();
            $row = mysqli_fetch_array($checklogin);
            $username = $row['User'];
            $email = $row['Email'];
            $name = $row['Name'];
            $surname = $row['Surname'];

            $_SESSION['Username'] = $username;
            $_SESSION['Email'] = $email;
            $_SESSION['Name'] = $name;
            $_SESSION['Surname'] = $surname;
            //session_id($_GET['PHPSESSID']);
            if (!isset($_SESSION['errors']))
                header('location: search_course.php');
            exit();         // Output a message and terminate the current script        
        }
    }
}

Warning: session_start(): The session id is too long or contains illegal characters, valid characters are a-z, A-Z, 0-9 and '-,' in C:\xampp\htdocs\swscholar\search_course.php on line 4

So I have renamed the start_session() function to my_start_session() which eliminates chars in the session id (I found one somewhere in there on StackOverflow) but I got an error in the search_course script that there isn't any existing $_SESSION set.