Nginx + PHP-FPM +多个服务器块

All,

I am attempting to configure nginx to serve up 3 separate directories and force the document root ($_SERVER['DOCUMENT_ROOT']) to be that directory.

Example: Give this url, http://127.0.0.1/helpdesk , we would like our $_SERVER['DOCUMENT_ROOT'] to be /var/www/helpdesk

Any ideas? Our block for /helpdesk looks like:

  location /helpdesk {      
     fastcgi_param DOCUMENT_ROOT /var/www/helpdesk;
     root /var/www/helpdesk
  }