Routes.php不适用于codeigniter中的子域重定向

Question

I have tried to redirect add page when url has sub domain name. but its not working.

My code in routes.php

if($_SERVER['HTTP_HOST']=='username.mysite.com'){
  $route['default_controller'] = 'advertisement/addAdvertisement';
}
else{
  $route['default_controller'] = 'Home';
}

$_SERVER['HTTP_HOST'] returns host name. if the host name is "username.mysite.com" (this is subdomain) means it will redirect to "mysite.com/advertisement/addAdvertisement". otherwise it will go to mysite.com only