Hi, sorry, i don't speak english, I will be very brief :D

I use this for php-cgi 404

location ~ \.php$ {
root public;
try_files $uri @404;
...
}

location @404 { return 404; break; }
error_page 404 /404.html;

in "http://localhost/index.php" work, in "http://localhost/forum/index.php" return 404 error... the folder exists and the index.php idem in /forum/ folder... any idea? :(