$https = (!empty($_SERVER['HTTPS']) && $_SERVER['HTTPS'] !== 'off') 
      || $_SERVER['SERVER_PORT'] == 443 
      || (!empty($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] === 'https');

if (!$https) {
    $url = 'https://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'];
    header("Location: $url", true, 301);
    exit;
}