Nginx subdomain display 404?

Code:

server {
        server_name *.mydomainname.info;
        if ( $host ~* (\b(?!www\b)\w+)\.\w+\.\w+ ) {
               set $subdomain /$1;
         }
        location / {
                root /home/nginx/domains/mydomainname.info/public$subdomain;
                index index.html index.php;
         }
        location ~ ^/(ads|d|e|skin)/
                {
                proxy_redirect        off;
                proxy_set_header    Host   www.mydomainname.info;
                proxy_pass      http://127.0.0.1:80;
                }
  include /usr/local/nginx/conf/staticfiles.conf;
  include /usr/local/nginx/conf/php.conf;
  include /usr/local/nginx/conf/drop.conf;
  include /usr/local/nginx/conf/vts_server.conf;

  if ($http_user_agent ~* "(YisouSpider|EasouSpider|EtaoSpider)") {
   return 403;
  }
    }

some subdomain is all right, but some are not.
It seems to me that if the index file is index.html, then subdomain.mydomain.info is all right
if the index file is index.php, then shows 404 error. Don't know why

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