if(isset($_GET["login"])){
    login_DB();
}
//funktionsdefinition des logins
function login_DB(){
if (empty ($_GET['username'])||empty ($_GET['password'])){
    $error = "Please enter a Username and a Password";
    echo $error;
}
else
{
    require("dbconnect.php");
    $username = $_GET['username'];
    $password = $_GET['password'];

    //Sql query zur datenbankabfrage
    $query = "select * from login where password='$password' AND username='$username'";
    $result = $con->query($query);
    var_dump($result);

    if ($result->num_rows > 0) {
                session_start();
                // Store Session Data
                $_SESSION['login_user']= $username;  // Initializing Session with value of PHP Variable
                echo $_SESSION['login_user'];
                echo"you are no logged in";
        }

        else {
            echo "Your username or password is wrong";
    }



}

<div id="loginregisterdiv">
        <form action="M133_A_140.php" method="get">
            <input type="text" name="username">
            <input type="password" name="password">
            <input type="submit" name="login" value="Login"    onclick="insert()" />
            <input type="submit" name="register" value="Registrieren"/>
        </form>
    </div>