amp-subdomain-paired-urls.php

<?php
/**
 * AMP Subdomain Paired URLs plugin file.
 *
 * @package   Google\AMP_Subdomain_Paired_URLs
 * @author    Weston Ruter, Google
 * @license   GPL-2.0-or-later
 * @copyright 2020 Google Inc.
 *
 * @wordpress-plugin
 * Plugin Name: AMP Subdomain Paired URLs
 * Plugin URI: https://gist.github.com/westonruter/67487d199f4cbc1968a30c60de121893
 * Description: Demonstration of how to configure AMP plugin v2.1 to use an "amp" subdomain for paired AMP URLs (in Transitional mode or Reader mode). Depends on <a href="https://github.com/ampproject/amp-wp/pull/5558">amp-wp#5558</a>.
 * Version: 0.2
 * Author: Weston Ruter, Google
 * Author URI: https://weston.ruter.net/
 * License: GNU General Public License v2 (or later)
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 * Gist Plugin URI: https://gist.github.com/westonruter/67487d199f4cbc1968a30c60de121893
 */

namespace AMP_Subdomain_Paired_URLs;

add_filter(
	'amp_custom_paired_url_structure',
	function () {
		require_once __DIR__ . '/SubdomainUrlStructure.php';
		return SubdomainUrlStructure::class;
	}
);

/*
 * In order for this to work properly, you must define the necessary cookie constants in the wp-config.php to include
 * subdomains. For example, if your site is located at https://wordpressdev.lndo.site then you may need to add this to
 * add this your wp-config.php:
 *
 *     define( 'COOKIE_DOMAIN', '.wordpressdev.lndo.site' );
 *     define( 'COOKIEHASH', md5( 'wordpressdev.lndo.site' ) );
 *     define( 'COOKIEPATH', '/' );
 *
 * If you do not do this, then you will not be able to preview AMP post changes or do paired browsing.
 */

/**
 * Check whether the current request is for an AMP page.
 *
 * @return bool Is an AMP request.
 */
function is_amp_request() {
	return function_exists( 'amp_is_request' ) && amp_is_request();
}

/**
 * Determine whether a URL is for AMP.
 *
 * @param string $url URL.
 * @return bool Whether the URL has the AMP subdomain.
 */
function has_amp_subdomain( $url ) {
	return (bool) preg_match(
		'/^amp\./',
		wp_parse_url( $url, PHP_URL_HOST )
	);
}

/**
 * Add amp subdomain to a URL.
 *
 * @param string $url URL.
 * @return string URL with amp subdomain.
 */
function add_amp_subdomain( $url ) {
	return preg_replace( '#(?<=://)(?!amp\.)#', 'amp.', $url );
}

/**
 * Remove amp subdomain from a URL.
 *
 * @param string $url URL which may have an amp subdomain.
 * @return string URL without amp subdomain.
 */
function remove_amp_subdomain( $url ) {
	return preg_replace( '#(?<=://)amp\.#', '', $url );
}

// Since the WP_HOME may have been modified so that the amp subdomain is an alias, make sure that the post canonical
// links get the subdomain removed on AMP pages.
add_filter(
	'get_canonical_url',
	function ( $canonical_url ) {
		if ( is_amp_request() ) {
			$canonical_url = remove_amp_subdomain( $canonical_url );
		}
		return $canonical_url;
	}
);

// Make sure that validation requests to the amp subdomain are allowed.
add_filter(
	'allowed_redirect_hosts',
	function ( $hosts ) {
		$hosts[] = wp_parse_url( add_amp_subdomain( home_url() ), PHP_URL_HOST );
		return $hosts;
	}
);

SubdomainUrlStructure.php

<?php
/**
 * Class SubdomainUrlStructure.
 *
 * @package Google\AMP_Path_Prefix_Paired_URLs
 */

namespace AMP_Subdomain_Paired_URLs;

use AmpProject\AmpWP\PairedUrlStructure;

/**
 * Descriptor for paired URL structures that use an AMP subdomain.
 */
class SubdomainUrlStructure extends PairedUrlStructure {

	/**
	 * Add amp subdomain to a URL.
	 *
	 * @param string $url URL (or path).
	 * @return string URL with amp subdomain.
	 */
	public function add_endpoint( $url ) {
		return add_amp_subdomain( $url );
	}

	/**
	 * Remove AMP path prefix from a URL.
	 *
	 * @param string $url URL (or path).
	 * @return string URL without amp subdomain.
	 */
	public function remove_endpoint( $url ) {
		return remove_amp_subdomain( $url );
	}
}

Admin URLs will need to be filtered to remove the AMP subdomain as well. Some items in the AMP admin bar menu may not resolve correctly, for example.

What if the subdomain doesn't have Wordpress installed does it still work with the main site. I know CNN did amp.cnn.com I would like to do amp.mydomain.com

@Wesley-Carnicle Unsure. This is only relevant when WordPress is installed on the subdomain and specifically when the subdomain is an alias of the main domain, and where the same WordPress install with the AMP plugin is on both.