Send additional HTTP headers to Nginx’s FastCGI

It’s not that often that I come across a useful, but undocumented feature in a major software application.  It happened recently, so I’ll document it here just for the future self.

For a particular setup, I had to send additional HTTP headers (let’s use X-GEOIP for this example) to the PHP-FPM, which was configured as a FastCGI backend in Nginx web server.  This StackOverflow thread suggested several solutions, but this one was the easiest and worked like a charm: use Nginx’s fastcgi_param directive AND prefix your variables with HTTP_.  For example:

location ~ \.php$ {
  fastcgi_param HTTP_X_GEOIP $geoip;
  ... other settings
}