Setting CF-IPCountry as a response header via PHP

This guide will show you how to create a simple module to capture the CF-IPCountry header incoming from Cloudflare (https://support.cloudflare.com/hc/en-us/articles/200168236-Configuring-Cloudflare-IP-Geolocation) and set it to the response headers using an Event Subscriber. 

Create a custom module using the following structure replacing cf_header and CfSubscriber with the names of your choice:

Edit the CfSubscriber.php file to contain the below, modifying the namespace and class name appropriately: 

<?php

namespace Drupal\cf_header\EventSubscriber; #<-- MODIFY THIS

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\Event\FilterResponseEvent;
use Symfony\Component\HttpFoundation\RequestStack;

/**
 * Class CfSubscriber. <-- MODIFY THIS
 */
class CfSubscriber implements EventSubscriberInterface { #<-- MODIFY THIS

  /**
   * Symfony\Component\HttpFoundation\RequestStack definition.
   *
   * @var \Symfony\Component\HttpFoundation\RequestStack
   */
  protected $requestStack;

  /**
   * Constructs a new CfSubscriber object. <-- MODIFY THIS
   *
   * @param \Symfony\Component\HttpFoundation\RequestStack $request_stack
   */
  public function __construct(RequestStack $request_stack) {
    $this->requestStack = $request_stack;
  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    $events['kernel.response'] = ['kernelResponse'];

    return $events;
  }

  /**
   * This method is called when the kernel.response is dispatched.
   *
   * @param \Symfony\Component\HttpKernel\Event\FilterResponseEvent $event
   *   The dispatched event.
   */
  public function kernelResponse(FilterResponseEvent $event) {
    $request = $this->requestStack->getCurrentRequest();
    $headers = $request->headers->get('CF-IPCountry');
    $response = $event->getResponse();
    $response->headers->set('CF-IPCountry', $headers);

  }

}

Edit cf_header.services.yaml to contain the below, modifying service name and class name appropriately: 

services:
  cf_header.default: # <-- MODIFY THIS
    class: Drupal\cf_header\EventSubscriber\CfSubscriber # <-- MODIFY THIS
    arguments: ['@request_stack']
    tags:
      - { name: event_subscriber }

You can now enable the module on your site which will capture the request header header and set it to the response.