Thanks Igor.
With your suggested configuration, can I use try_files as listed below?

location / {
root /var/www/html;
try_files $uri $uri/ /index.php;
}

location /forum/ {
try_files $uri $uri/ /forum/data.php$args;
}

location ~ \.php$ {
fastcgi_pass php;
include fastcgi.conf;
}