php redirect to page with message

By the time the redirect happens and the PHP script depicted by $location is executed, $message variable would have been long gone.

To tackle this, you need to pass your message in your location header, using GET variable:

header("Location: $location?message=success");

And

if(!empty($_GET['message'])) {
    $message = $_GET['message'];
// rest of your code

You could also have a look into sessions

session_start();
$_SESSION['message'] = 'success';
header("Location: $location");

then in the destination script:

session_start();
if(!empty($_SESSION['message'])) {
   $message = $_SESSION['message'];
   // rest of your code

Variables cease to exist after the script ends. Each separate request, each separate PHP script invocation is an entirely new context with no data from any other invocation.

Use sessions to persist data.

you can use sessions

if (mysqli_affected_rows($link) == 1) 
{
   //succes         
    $_SESSION['message'] = 'succes';
    redirect_to('index.php');
}

and on index

if (!empty($_SESSION['message'])) {
    echo '<p class="message"> '.$_SESSION['message'].'</p>';
    unset($_SESSION['message']);
}