// Install info.:
// Copy and paste these lines into your default index.php or
// the file that get's called if a visitor comes on your 
// website...
// read the host from the server environment
$host = $_SERVER["HTTP_HOST"];
// fix host name - we never now... ;-)
$host = strtolower($host);
$host = trim($host);
// This is important: 
// Webbrowsers like Firefox are doing their request without
// the port number like "www.jb51.net" but some other 
// applications send host names like "www.jb51.net:80" 
$host = str_replace(':80', '', $host);
$host = trim($host);
// if the host is not starting with www. redirect the 
// user to the same URL but with www :-)
if ($host != 'www.jb51.net'){
  // You an also change the "!=" to "==", if you want to force 
  // the user to use the domain name without the www. 
  // send status header, so that search engines or other services
  // detect that this is a permanent redirect and not a temporary
  header('HTTP/1.1 301 Moved Permanently');
  // read the URL the user requested:
  $url = isset($_SERVER["REQUEST_URI"]) "REQUEST_URI"] : '';
  // redirect the user to the new destination:
  header('Location: https://www.jb51.net' . $url);
  // Convert "special" chars -- cause we never now... ;-)
  $url = htmlspecialchars($url);
  // "fallback" link, if the browser is not supporting header redirects
  print '<a href="https://www.jb51.net' . $url.'">Please click here</a>';
  // stop the script execution here
  exit;
}
// If the domain is www.jb51.net then go on with your PHP code 
// of with your website...
// BTW: You need to replace jb51.net trough your own domain :-D