amp-auto-image-lightbox.php

<?php
/**
 * AMP Auto Image Lightbox plugin bootstrap.
 *
 * @package   Google\AMP_Auto_Image_Lightbox
 * @author    Weston Ruter, Google
 * @license   GPL-2.0-or-later
 * @copyright 2020 Google Inc.
 *
 * @wordpress-plugin
 * Plugin Name: AMP Auto Image Lightbox
 * Plugin URI: https://gist.github.com/westonruter/
 * Description: For images which are contained by a link to the same image file, automatically remove the link and put a lightbox attribute on the image to use amp-lightbox-gallery instead.
 * Version: 0.1.1
 * 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/fefdcda37df7bde61c7b0f690fafc01c
 */

namespace Google\AMP_Auto_Image_Lightbox;

/**
 * Filter sanitizers.
 *
 * @param array $sanitizers Sanitizers.
 * @return array Sanitizers.
 */
function filter_sanitizers( $sanitizers ) {
	if ( version_compare( AMP__VERSION, '1.5', '<=' ) ) {
		return $sanitizers;
	}

	require_once __DIR__ . '/class-sanitizer.php';

	return array_merge(
		[
			__NAMESPACE__ . '\Sanitizer' => [],
		],
		$sanitizers
	);
}
add_filter( 'amp_content_sanitizers', __NAMESPACE__ . '\filter_sanitizers' );

class-sanitizer.php

<?php
/**
 * Sanitizer
 *
 * @package Google\AMP_Auto_Image_Lightbox
 */

namespace Google\AMP_Auto_Image_Lightbox;

use AMP_Base_Sanitizer;
use DOMElement;

/**
 * Class Sanitizer
 */
class Sanitizer extends AMP_Base_Sanitizer {

	/**
	 * Sanitize.
	 */
	public function sanitize() {

		/**
		 * Element.
		 *
		 * @var DOMElement $img
		 */
		foreach ( $this->dom->xpath->query( '//a[ @href ]/img[ @src = parent::a/@href ]' ) as $img ) {
			$link = $img->parentNode;
			$link->parentNode->replaceChild( $img, $link );
			$img->setAttribute( 'lightbox', '' );
		}
	}
}

For context, see https://wordpress.org/support/topic/lightbox-not-working-on-amp-pages/

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

@westonruter (FYI) This code does not work on WP 5.9. It's like it doesn't use this sanitizer at all.

In the xpath I replaced amp-img with img and it works. It looks like this sanitizer was executing before AMP replaced img with amp-img.

<?php
/**
 * Sanitizer
 *
 * @package Google\AMP_Auto_Image_Lightbox
 */

namespace Google\AMP_Auto_Image_Lightbox;

use AMP_Base_Sanitizer;
use DOMElement;

/**
 * Class Sanitizer
 */
class Sanitizer extends AMP_Base_Sanitizer {

	/**
	 * Sanitize.
	 */
	public function sanitize() {

		/**
		 * Element.
		 *
		 * @var DOMElement $img
		 */
		foreach ( $this->dom->xpath->query( '//a[ @href ]/img[ @src = parent::a/@href ]' ) as $img ) {
			$link = $img->parentNode;
			$link->parentNode->replaceChild( $img, $link );
			$img->setAttribute( 'lightbox', '' );
		}
	}
}

@rafaucau Thanks, updated. This will also work out better in the future as amp-img is being deprecated. The img element will be used by default as of ampproject/amp-wp#6805.

The reason why it didn't work anymore is that in 2.2 the builtin sanitizers are now always run in a set order after any user-supplied transformers.