amp-path-prefix-paired-urls.php

<?php
/**
 * AMP Path Prefix Paired URLs plugin file.
 *
 * @package   Google\AMP_Path_Prefix_Paired_URLs
 * @author    Weston Ruter, Google
 * @license   GPL-2.0-or-later
 * @copyright 2020 Google Inc.
 *
 * @wordpress-plugin
 * Plugin Name: AMP Path Prefix Paired URLs
 * Plugin URI: https://gist.github.com/westonruter/f881658cb1bc5a03cc432c8e2b89102a
 * Description: Demonstration of how to configure AMP plugin v2.1 to use an "amp" path prefix 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: 0.3
 * 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/f881658cb1bc5a03cc432c8e2b89102a
 */

namespace AMP_Path_Prefix_Paired_URLs;

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

PrefixUrlStructure.php

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

namespace AMP_Path_Prefix_Paired_URLs;

use AmpProject\AmpWP\PairedUrlStructure;

/**
 * Descriptor for paired URL structures that start in /amp/ path prefix.
 */
class PrefixUrlStructure extends PairedUrlStructure {

	/**
	 * Add amp subdomain to a URL.
	 *
	 * @param string $url URL (or REQUEST_URI).
	 * @return string URL with AMP path prefix.
	 */
	public function add_endpoint( $url ) {
		if ( $this->has_endpoint( $url ) ) {
			return $url;
		}
		$slug = amp_get_slug();
		return preg_replace(
			'#^((\w+:)?//[^/]+)?/#',
			"$1/{$slug}/",
			$url
		);
	}

	/**
	 * Remove AMP path prefix from a URL.
	 *
	 * @param string $url URL (or REQUEST_URI).
	 * @return string URL without AMP path prefix.
	 */
	public function remove_endpoint( $url ) {
		return preg_replace(
			sprintf(
				'#^((\w+:)?//[^/]+)?/%s/#',
				preg_quote( amp_get_slug(), '#' )
			),
			'$1/',
			$url
		);
	}
}

Installation instructions: https://gist.github.com/westonruter/6110fbc4bef0c4b8c021a112012f7e9c