Example sanitizer to add a custom attribute to links in a story

web-stories-link-sanitizer-custom-attribute.php

<?php
/**
 * Plugin Name: Web Stories Link sanitizer example
 * Description: Example sanitizer to add a custom attribute to links in a story
 * Author: Pascal Birchler
 * Author URI: https://pascalbirchler.com/
 * Version: 0.0.1
 * Requires at least: 6.1
 * Requires PHP: 7.4
 * License: Apache License 2.0
 * License URI: https://www.apache.org/licenses/LICENSE-2.0
 *
 * @copyright 2023 Google LLC
 * @license   https://www.apache.org/licenses/LICENSE-2.0 Apache License 2.0
 */


if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

add_action(
	'init',
	static function() {
		if ( ! class_exists( '\Google\Web_Stories_Dependencies\AMP_Base_Sanitizer' ) ) {
			return;
		}

		add_filter(
			'web_stories_amp_sanitizers',
			static function( $sanitizers ) {
				$sanitizers[ Your_Website_Example_AMP_Sanitizer::class ] = [];

				return $sanitizers;
			}
		);

		class Your_Website_Example_AMP_Sanitizer extends \Google\Web_Stories_Dependencies\AMP_Base_Sanitizer
		{
			/**
			 * Sanitize the HTML contained in the DOMDocument received by the constructor.
			 */
			public function sanitize(): void
			{
				$links = $this->dom->body->getElementsByTagName( 'a' );

				/**
				 * The <a> element
				 *
				 * @var DOMElement $link The <a> element
				 */
				foreach ( $links as $link ) {
					$url = $link->getAttribute( 'href' );

					$is_relative_link = str_starts_with( $url, '#' );

					if ( ! $is_relative_link && ! $link->getAttribute( 'target' ) ) {
						$link->setAttribute( 'target', '_blank' );
					}

					$is_link_to_same_origin = str_starts_with( $url, home_url() ) || $is_relative_link;

					if ( $is_link_to_same_origin ) {
						$link->setAttribute( 'data-vars-click-url', $url );
					}
				}
			}
		}
	}
);