inloggen werkt niet

if($stmt->rowCount() == 1){
                        if($row = $stmt->fetch()){
                            $UserID = $row["UserID"];
                            $username = $row["Username"];
                            $hashed_password = $row["Password"];
                            if(password_verify($password, $hashed_password)){
                                // password is correct, so start new session
                                session_start();

                                //store data in session variables
                                $_SESSION["loggedin"] = true;
                                $_SESSION["UserID"] = $UserID;
                                $_SESSION["username"] = $username;

                                //redirect user to welcome page
                                header("location: welcome.php");
                            }
                            else{
                                //invalid password, display error message
                                $login_err = "Invalid username or password.";
                            }