php if current url is subdomain, redirect to main domain

function remove_subdomain_from_url(){
  if(!empty($_SERVER['HTTP_HOST']) && $_SERVER['HTTP_HOST'] != 'localhost'){
    $host = $_SERVER['HTTP_HOST'];
    if(strpos('www.', $host) === 0){
      $host = substr($host, 4);
    }
    if(strpos($host, '.') !== false){
      $parts = explode('.', $host);
      if(count($parts) > 2){
        unset($parts[0]);
        $host = implode('.', $parts);
        header('Location: http://' . $host . $_SERVER['REQUEST_URI']);
      }
    }
  }
}