redirect to a PHP page with header

Good day.

I have a script which after a user signs in sends it to the control panel of their account through a link that has to be clicked, everything works great, but I think it would be better to automatically redirect to the control panel and so not having to click on the mentioned link, the problem is that it does not redirect with header, the following error comes out

  

Warning: Can not modify header information - headers already sent by   (output started at   /home2/gabriel/tecflucol.com/sistema/PHP/checklogin.php:4) in   /home2/softwareiii.com/sistema/PHP/checklogin.php on line 26

How could I solve it and redirect the page automatically?

Here's the code:

<?php
    session_start();
?>

<?php
    include('../PHP/conectar.php');

    $username = $_POST['username'];
    $password = $_POST['password'];

    $sql = "SELECT * FROM 'usuarios' WHERE 'usuario'='$username';";
    $result = $conexion->query($sql);

    if ($result->num_rows > 0)
    {
        $row = $result->fetch_array(MYSQLI_ASSOC);
    }

    if (password_verify($password, $row['contrasena']))
    {
        $_SESSION['loggedin'] = true;
        $_SESSION['usuario'] = $username;
        $_SESSION['start'] = time();
        $_SESSION['expire'] = $_SESSION['start'] + (5 * 800000);

        //rediriguir automanticamente
        header('Location: panel_de_control.php');

        //estas dos lineas funcionan bien pero la idea es suprimirlas
        //echo "Bienvenido(a)! ".$_SESSION['usuario'];
        //echo "<br><br><a href=panel_de_control.php>PANEL DE CONTROL</a>";
    }
    else
    {
      echo "Usuario o contraseña estan incorrectos.";
      echo "<br><a href='../login.html'>Volver a Intentarlo</a>";
    }
     mysqli_close($conexion);
?>