php if current url is subdomain name 'media', redirect to main domain

<?php
  $host = $_SERVER['HTTP_HOST']; // example: dev.example.com
  $host = explode('.', $host);
  $host = $host[0];
  if($host == 'media'){
    header('Location: http://www.example.com', true, 301);
    exit();
  }
?>