WordPress mini plugin for AMP that transforms all images linking to themselves to open in an `amp-image-lightbox` overlaying the website content. For docs on the component, see https://amp.dev/documentation/components/amp-image-lightbox/

amp-image-lightbox-links.php

<?php
/**
 * AMP Image Lightbox Links plugin file.
 *
 * @package   AMP_Image_Lightbox_Links
 * @author    Felix Arntz, Google
 * @license   GPL-2.0-or-later
 * @copyright 2019 Google Inc.
 *
 * @wordpress-plugin
 * Plugin Name: AMP Image Lightbox Links
 * Description: Modifies all images that link to themselves to open in an amp-image-lightbox.
 * Plugin URI:  https://gist.github.com/felixarntz/01f7099ec718d73fb25ff81fc408111b
 * Version:     0.1.0
 * Author:      Felix Arntz, Google
 * Author URI:  https://felix-arntz.me
 * License:     GNU General Public License v2 (or later)
 * License URI: https://www.gnu.org/licenses/gpl-2.0.html
 */

namespace AMP_Image_Lightbox_Links;

add_filter(
	'amp_content_sanitizers',
	function( array $sanitizers ) {
		if ( ! class_exists( __NAMESPACE__ . '\\Sanitizer' ) ) {
			require_once plugin_dir_path( __FILE__ ) . 'class-sanitizer.php';
		}

		$sanitizers[ __NAMESPACE__ . '\\Sanitizer' ] = [];
		return $sanitizers;
	}
);

add_action(
	'wp_head',
	function() {
		echo '<style>amp-img[role="button"] { cursor: pointer; }</style>';
	}
);

class-sanitizer.php

<?php
/**
 * Class: AMP_Image_Lightbox_Links\\Sanitizer
 *
 * @package   AMP_Image_Lightbox_Links
 * @author    Felix Arntz, Google
 * @license   GPL-2.0-or-later
 * @copyright 2019 Google Inc.
 */

namespace AMP_Image_Lightbox_Links;

use AMP_Base_Sanitizer;
use DOMXPath;

/**
 * Class sanitizing images linking to themselves to use the amp-image-lightbox component.
 */
class Sanitizer extends AMP_Base_Sanitizer {

	/**
	 * XPath for the DOMDocument.
	 *
	 * @var DOMXPath
	 */
	protected $xpath;

	/**
	 * Sanitizes the HTML contained in the DOMDocument.
	 */
	public function sanitize() {
		$this->xpath = new DOMXPath( $this->dom );

		$this->add_image_lightbox();
	}

	/**
	 * Replaces image links to themselves with the amp-image-lightbox component.
	 */
	protected function add_image_lightbox() {
		$linked_images = $this->xpath->query( '//a[@href]/child::amp-img' );
		if ( ! $linked_images->length ) {
			return;
		}

		$count = 0;
		foreach ( $linked_images as $linked_image ) {
			$link = $linked_image->parentNode;
			$href = preg_replace( '/-(\d+)x(\d+)\.(jpg|png|gif)$/', '.$3', $link->getAttribute( 'href' ) );
			$src  = preg_replace( '/-(\d+)x(\d+)\.(jpg|png|gif)$/', '.$3', $linked_image->getAttribute( 'src' ) );
			if ( $href !== $src ) {
				continue;
			}
			$linked_image->setAttribute( 'on', 'tap:linked-image-lightbox.open' );
			$linked_image->setAttribute( 'role', 'button' );
			$linked_image->setAttribute( 'tabindex', '0' );
			$link->parentNode->replaceChild( $linked_image, $link );
			$count++;
		}

		if ( ! $count ) {
			return;
		}

		$lightbox = $this->dom->createElement( 'amp-image-lightbox' );
		$lightbox->setAttribute( 'id', 'linked-image-lightbox' );
		$lightbox->setAttribute( 'layout', 'nodisplay' );
		$lightbox->setAttribute( 'data-close-button-aria-label', _x( 'Close', 'lightbox', 'amp-image-lightbox-links' ) );
		$this->dom->getElementsByTagName( 'body' )->item( 0 )->appendChild( $lightbox );
	}
}