/*
 * Add a specific Cloudflare edge cache HTTP header to each HTTP response
 * preventing defined pages from being cached at the edge. This is done
 * selectively since some pages require dynamic content based on external
 * factors such as geographic location of the request. All other static
 * content will continue to be cached (i.e. JavaScript, CSS, images).
 *
 * To control the caching behaviour at the edge add a custom metadata field
 * to the page:
 *
 * field name  : cache_option
 * field value : no-cache|location-cache
 * 
 */ 
add_filter('wp_headers', 'no_edge_cache');
function no_edge_cache($headers) {
	// Using get_the_ID() is not possible within this hook because the global
	// $post variable will not have been set by the Wordpress run loop. This
	// code recreates the current request URL and uses a global Wordpress function
	// to fetch the post ID.
	$url = $_SERVER['REQUEST_SCHEME'] . '://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'];
	$current_post_id = url_to_postid( $url );

	switch (get_metadata('post', $current_post_id, 'cache_option', true)) {
	case 'no-cache':
		$headers['cf-edge-cache'] = 'no-cache';
		break;
	case 'location-cache':
		$headers['Vary'] = 'Cf-Ipcountry';
		break;
	}

	return $headers;
}