amp.php

<?php
/**
 * Plugin Name: AMP
 * Description: Enable AMP on your WordPress site, the WordPress way.
 * Plugin URI: https://amp-wp.org
 * Author: AMP Project Contributors
 * Author URI: https://github.com/ampproject/amp-wp/graphs/contributors
 * Version: 1.3-beta1
 * Text Domain: amp
 * Domain Path: /languages/
 * License: GPLv2 or later
 *
 * @package AMP
 */

define( 'AMP__FILE__', __FILE__ );
define( 'AMP__DIR__', dirname( __FILE__ ) );
define( 'AMP__VERSION', '1.3-beta1' );

/**
 * Errors encountered while loading the plugin.
 *
 * This has to be a global for the same of PHP 5.2.
 *
 * @var WP_Error $_amp_load_errors
 */
global $_amp_load_errors;

$_amp_load_errors = new WP_Error();

if ( version_compare( phpversion(), '5.4', '<' ) ) {
	$_amp_load_errors->add(
		'insufficient_php_version',
		sprintf(
			/* translators: %s: required PHP version */
			__( 'The AMP plugin requires PHP %s. Please contact your host to update your PHP version.', 'amp' ),
			'5.4+'
		)
	);
}

// See composer.json for this list.
$_amp_required_extensions = array(
	// Required by FasterImage.
	'curl'   => array(
		'functions' => array(
			'curl_close',
			'curl_error',
			'curl_exec',
			'curl_init',
			'curl_setopt',
		),
	),
	'dom'    => array(
		'classes' => array(
			'DOMAttr',
			'DOMComment',
			'DOMDocument',
			'DOMElement',
			'DOMNode',
			'DOMNodeList',
			'DOMXPath',
		),
	),
	// Required by PHP-CSS-Parser.
	'iconv'  => array(
		'functions' => array( 'iconv' ),
	),
	'libxml' => array(
		'functions' => array( 'libxml_use_internal_errors' ),
	),
	'spl'    => array(
		'functions' => array( 'spl_autoload_register' ),
	),
);
$_amp_missing_extensions = array();
$_amp_missing_classes    = array();
$_amp_missing_functions  = array();
foreach ( $_amp_required_extensions as $_amp_required_extension => $_amp_required_constructs ) {
	if ( ! extension_loaded( $_amp_required_extension ) ) {
		$_amp_missing_extensions[] = "<code>$_amp_required_extension</code>";
	} else {
		foreach ( $_amp_required_constructs as $_amp_construct_type => $_amp_constructs ) {
			switch ( $_amp_construct_type ) {
				case 'functions':
					foreach ( $_amp_constructs as $_amp_construct ) {
						if ( ! function_exists( $_amp_construct ) ) {
							$_amp_missing_functions[] = "<code>$_amp_construct</code>";
						}
					}
					break;
				case 'classes':
					foreach ( $_amp_constructs as $_amp_construct ) {
						if ( ! class_exists( $_amp_construct ) ) {
							$_amp_missing_classes[] = "<code>$_amp_construct</code>";
						}
					}
					break;
			}
		}
		unset( $_amp_construct_type, $_amp_constructs );
	}
}
if ( count( $_amp_missing_extensions ) > 0 ) {
	$_amp_load_errors->add(
		'missing_extension',
		sprintf(
			/* translators: %s is list of missing extensions */
			_n(
				'The following PHP extension is missing: %s. Please contact your host to finish installation.',
				'The following PHP extensions are missing: %s. Please contact your host to finish installation.',
				count( $_amp_missing_extensions ),
				'amp'
			),
			implode( ', ', $_amp_missing_extensions )
		)
	);
}
if ( count( $_amp_missing_classes ) > 0 ) {
	$_amp_load_errors->add(
		'missing_class',
		sprintf(
			/* translators: %s is list of missing extensions */
			_n(
				'The following PHP class is missing: %s. Please contact your host to finish installation.',
				'The following PHP classes are missing: %s. Please contact your host to finish installation.',
				count( $_amp_missing_classes ),
				'amp'
			),
			implode( ', ', $_amp_missing_classes )
		)
	);
}
if ( count( $_amp_missing_functions ) > 0 ) {
	$_amp_load_errors->add(
		'missing_class',
		sprintf(
			/* translators: %s is list of missing extensions */
			_n(
				'The following PHP function is missing: %s. Please contact your host to finish installation.',
				'The following PHP functions are missing: %s. Please contact your host to finish installation.',
				count( $_amp_missing_functions ),
				'amp'
			),
			implode( ', ', $_amp_missing_functions )
		)
	);
}

unset( $_amp_required_extensions, $_amp_missing_extensions, $_amp_required_constructs, $_amp_missing_classes, $_amp_missing_functions, $_amp_required_extension, $_amp_construct_type, $_amp_construct, $_amp_constructs );

// DEV_CODE. This block of code is removed during the build process.
if ( ! file_exists( AMP__DIR__ . '/vendor/autoload.php' ) || ! file_exists( AMP__DIR__ . '/vendor/sabberworm/php-css-parser' ) || ! file_exists( AMP__DIR__ . '/assets/js/amp-block-editor.js' ) ) {
	$_amp_load_errors->add(
		'build_required',
		sprintf(
			/* translators: %s: composer install && npm install && npm run build */
			__( 'You appear to be running the AMP plugin from source. Please do %s to finish installation.', 'amp' ), // phpcs:ignore WordPress.Security.EscapeOutput
			'<code>composer install &amp;&amp; npm install &amp;&amp; npm run build</code>'
		)
	);
}

/**
 * Displays an admin notice about why the plugin is unable to load.
 *
 * @since 1.1.2
 * @global WP_Error $_amp_load_errors
 */
function _amp_show_load_errors_admin_notice() {
	global $_amp_load_errors;
	?>
	<div class="notice notice-error">
		<p>
			<strong><?php esc_html_e( 'AMP plugin unable to initialize.', 'amp' ); ?></strong>
			<ul>
			<?php foreach ( array_keys( $_amp_load_errors->errors ) as $error_code ) : ?>
				<?php foreach ( $_amp_load_errors->get_error_messages( $error_code ) as $message ) : ?>
					<li>
						<?php echo wp_kses_post( $message ); ?>
					</li>
				<?php endforeach; ?>
			<?php endforeach; ?>
			</ul>
		</p>
	</div>
	<?php
}

// Abort if dependencies are not satisfied.
if ( ! empty( $_amp_load_errors->errors ) ) {
	add_action( 'admin_notices', '_amp_show_load_errors_admin_notice' );

	if ( ( defined( 'WP_CLI' ) && WP_CLI ) || 'true' === getenv( 'CI' ) || 'cli' === PHP_SAPI ) {
		$messages = array( __( 'AMP plugin unable to initialize.', 'amp' ) );
		foreach ( array_keys( $_amp_load_errors->errors ) as $error_code ) {
			$messages = array_merge( $messages, $_amp_load_errors->get_error_messages( $error_code ) );
		}
		$message = implode( "\n * ", $messages );
		$message = str_replace( array( '<code>', '</code>' ), '`', $message );
		$message = html_entity_decode( $message, ENT_QUOTES );

		if ( ! class_exists( 'WP_CLI' ) ) {
			echo "$message\n"; // phpcs:disable WordPress.Security.EscapeOutput.OutputNotEscaped

			exit( 1 );
		}

		WP_CLI::warning( $message );
	}

	return;
}


/**
 * Print admin notice if plugin installed with incorrect slug (which impacts WordPress's auto-update system).
 *
 * @since 1.0
 */
function _amp_incorrect_plugin_slug_admin_notice() {
	$actual_slug = basename( AMP__DIR__ );
	?>
	<div class="notice notice-warning">
		<p>
			<?php
			echo wp_kses_post(
				sprintf(
					/* translators: %1$s is the current directory name, and %2$s is the required directory name */
					__( 'You appear to have installed the AMP plugin incorrectly. It is currently installed in the <code>%1$s</code> directory, but it needs to be placed in a directory named <code>%2$s</code>. Please rename the directory. This is important for WordPress plugin auto-updates.', 'amp' ),
					$actual_slug,
					'amp'
				)
			);
			?>
		</p>
	</div>
	<?php
}
if ( 'amp' !== basename( AMP__DIR__ ) ) {
	add_action( 'admin_notices', '_amp_incorrect_plugin_slug_admin_notice' );
}

/**
 * Print admin notice if the Xdebug extension is loaded.
 *
 * @since 1.3
 */
function _amp_xdebug_admin_notice() {
	?>
	<div class="notice notice-warning">
		<p>
			<?php
			esc_html_e(
				'Your server currently has the Xdebug PHP extension loaded. This can cause some of the AMP plugin\'s processes to timeout depending on your system resources and configuration. Please deactivate Xdebug for the best experience.',
				'amp'
			);
			?>
		</p>
	</div>
	<?php
}
if ( extension_loaded( 'xdebug' ) ) {
	add_action( 'admin_notices', '_amp_xdebug_admin_notice' );
}

require_once AMP__DIR__ . '/includes/class-amp-autoloader.php';
AMP_Autoloader::register();

require_once AMP__DIR__ . '/back-compat/back-compat.php';
require_once AMP__DIR__ . '/includes/amp-helper-functions.php';
require_once AMP__DIR__ . '/includes/admin/functions.php';

register_activation_hook( __FILE__, 'amp_activate' );

/**
 * Handle activation of plugin.
 *
 * @since 0.2
 */
function amp_activate() {
	amp_after_setup_theme();
	if ( ! did_action( 'amp_init' ) ) {
		amp_init();
	}
	flush_rewrite_rules();
}

register_deactivation_hook( __FILE__, 'amp_deactivate' );

/**
 * Handle deactivation of plugin.
 *
 * @since 0.2
 */
function amp_deactivate() {
	// We need to manually remove the amp endpoint.
	global $wp_rewrite;
	foreach ( $wp_rewrite->endpoints as $index => $endpoint ) {
		if ( amp_get_slug() === $endpoint[1] ) {
			unset( $wp_rewrite->endpoints[ $index ] );
			break;
		}
	}

	flush_rewrite_rules( false );
}

/*
 * Register AMP scripts regardless of whether AMP is enabled or it is the AMP endpoint
 * for the sake of being able to use AMP components on non-AMP documents ("dirty AMP").
 */
add_action( 'wp_default_scripts', 'amp_register_default_scripts' );

// Ensure async and custom-element/custom-template attributes are present on script tags.
add_filter( 'script_loader_tag', 'amp_filter_script_loader_tag', PHP_INT_MAX, 2 );

// Ensure crossorigin=anonymous is added to font links.
add_filter( 'style_loader_tag', 'amp_filter_font_style_loader_tag_with_crossorigin_anonymous', 10, 4 );

/**
 * Set up AMP.
 *
 * This function must be invoked through the 'after_setup_theme' action to allow
 * the AMP setting to declare the post types support earlier than plugins/theme.
 *
 * @since 0.6
 */
function amp_after_setup_theme() {
	amp_get_slug(); // Ensure AMP_QUERY_VAR is set.

	/**
	 * Filters whether AMP is enabled on the current site.
	 *
	 * Useful if the plugin is network activated and you want to turn it off on select sites.
	 *
	 * @since 0.2
	 */
	if ( false === apply_filters( 'amp_is_enabled', true ) ) {
		return;
	}

	add_action( 'init', 'amp_init', 0 ); // Must be 0 because widgets_init happens at init priority 1.
}
add_action( 'after_setup_theme', 'amp_after_setup_theme', 5 );

/**
 * Init AMP.
 *
 * @since 0.1
 */
function amp_init() {

	/**
	 * Triggers on init when AMP plugin is active.
	 *
	 * @since 0.3
	 */
	do_action( 'amp_init' );

	add_filter( 'allowed_redirect_hosts', array( 'AMP_HTTP', 'filter_allowed_redirect_hosts' ) );
	AMP_HTTP::purge_amp_query_vars();
	AMP_HTTP::send_cors_headers();
	AMP_HTTP::handle_xhr_request();
	AMP_Theme_Support::init();
	AMP_Validation_Manager::init();
	AMP_Service_Worker::init();
	add_action( 'admin_init', 'AMP_Options_Manager::register_settings' );
	add_action( 'wp_loaded', 'amp_add_options_menu' );
	add_action( 'wp_loaded', 'amp_admin_pointer' );
	add_action( 'wp_loaded', 'amp_post_meta_box' ); // Used in both Website and Stories experiences.

	if ( AMP_Options_Manager::is_website_experience_enabled() ) {
		add_rewrite_endpoint( amp_get_slug(), EP_PERMALINK );
		AMP_Post_Type_Support::add_post_type_support();
		add_action( 'init', array( 'AMP_Post_Type_Support', 'add_post_type_support' ), 1000 ); // After post types have been defined.
		add_action( 'parse_query', 'amp_correct_query_when_is_front_page' );
		add_action( 'admin_bar_menu', 'amp_add_admin_bar_view_link', 100 );
		add_action( 'wp_loaded', 'amp_editor_core_blocks' );
		add_filter( 'request', 'amp_force_query_var_value' );

		// Add actions for reader mode templates.
		add_action( 'wp', 'amp_maybe_add_actions' );

		// Redirect the old url of amp page to the updated url.
		add_filter( 'old_slug_redirect_url', 'amp_redirect_old_slug_to_new_url' );
	}

	if ( AMP_Options_Manager::is_stories_experience_enabled() ) {
		AMP_Story_Post_Type::register();
	}

	// Does its own is_stories_experience_enabled() check.
	add_action( 'wp_loaded', 'amp_story_templates' );

	if ( defined( 'WP_CLI' ) && WP_CLI ) {
		if ( class_exists( 'WP_CLI\Dispatcher\CommandNamespace' ) ) {
			WP_CLI::add_command( 'amp', 'AMP_CLI_Namespace' );
		}

		WP_CLI::add_command( 'amp validation', 'AMP_CLI_Validation_Command' );
	}

	/*
	 * Broadcast plugin updates.
	 * Note that AMP_Options_Manager::get_option( 'version', '0.0' ) cannot be used because
	 * version was new option added, and in that case default would never be used for a site
	 * upgrading from a version prior to 1.0. So this is why get_option() is currently used.
	 */
	$options     = get_option( AMP_Options_Manager::OPTION_NAME, array() );
	$old_version = isset( $options['version'] ) ? $options['version'] : '0.0';
	if ( AMP__VERSION !== $old_version ) {
		/**
		 * Triggers when after amp_init when the plugin version has updated.
		 *
		 * @param string $old_version Old version.
		 */
		do_action( 'amp_plugin_update', $old_version );
		AMP_Options_Manager::update_option( 'version', AMP__VERSION );
	}
}

/**
 * Make sure the `amp` query var has an explicit value.
 *
 * This avoids issues when filtering the deprecated `query_string` hook.
 *
 * @since 0.3.3
 *
 * @param array $query_vars Query vars.
 * @return array Query vars.
 */
function amp_force_query_var_value( $query_vars ) {
	if ( isset( $query_vars[ amp_get_slug() ] ) && '' === $query_vars[ amp_get_slug() ] ) {
		$query_vars[ amp_get_slug() ] = 1;
	}
	return $query_vars;
}

/**
 * Conditionally add AMP actions or render the transitional mode template(s).
 *
 * If the request is for an AMP page and this is in 'canonical mode,' redirect to the non-AMP page.
 * It won't need this plugin's template system, nor the frontend actions like the 'rel' link.
 *
 * @deprecated This function is not used when 'amp' theme support is added.
 * @global WP_Query $wp_query
 * @since 0.2
 * @return void
 */
function amp_maybe_add_actions() {

	// Short-circuit when theme supports AMP, as everything is handled by AMP_Theme_Support.
	if ( current_theme_supports( AMP_Theme_Support::SLUG ) ) {
		return;
	}

	// The remaining logic here is for transitional mode running in themes that don't support AMP, the template system in AMP<=0.6.
	global $wp_query;
	if ( ! ( is_singular() || $wp_query->is_posts_page ) || is_feed() ) {
		return;
	}

	if ( is_singular( AMP_Story_Post_Type::POST_TYPE_SLUG ) ) {
		return;
	}

	$is_amp_endpoint = is_amp_endpoint();

	/**
	 * Queried post object.
	 *
	 * @var WP_Post $post
	 */
	$post = get_queried_object();
	if ( ! post_supports_amp( $post ) ) {
		if ( $is_amp_endpoint ) {
			/*
			 * Temporary redirect is used for admin users because reader mode and AMP support can be enabled by user at any time,
			 * so they will be able to make AMP available for this URL and see the change without wrestling with the redirect cache.
			 */
			wp_safe_redirect( get_permalink( $post->ID ), current_user_can( 'manage_options' ) ? 302 : 301 );
			exit;
		}
		return;
	}

	if ( $is_amp_endpoint ) {

		// Prevent infinite URL space under /amp/ endpoint.
		global $wp;
		$path_args = [];
		wp_parse_str( $wp->matched_query, $path_args );
		if ( isset( $path_args[ amp_get_slug() ] ) && '' !== $path_args[ amp_get_slug() ] ) {
			wp_safe_redirect( amp_get_permalink( $post->ID ), 301 );
			exit;
		}

		amp_prepare_render();
	} else {
		amp_add_frontend_actions();
	}
}

/**
 * Fix up WP_Query for front page when amp query var is present.
 *
 * Normally the front page would not get served if a query var is present other than preview, page, paged, and cpage.
 *
 * @since 0.6
 * @see WP_Query::parse_query()
 * @link https://github.com/WordPress/wordpress-develop/blob/0baa8ae85c670d338e78e408f8d6e301c6410c86/src/wp-includes/class-wp-query.php#L951-L971
 *
 * @param WP_Query $query Query.
 */
function amp_correct_query_when_is_front_page( WP_Query $query ) {
	$is_front_page_query = (
		$query->is_main_query()
		&&
		$query->is_home()
		&&
		// Is AMP endpoint.
		false !== $query->get( amp_get_slug(), false )
		&&
		// Is query not yet fixed uo up to be front page.
		! $query->is_front_page()
		&&
		// Is showing pages on front.
		'page' === get_option( 'show_on_front' )
		&&
		// Has page on front set.
		get_option( 'page_on_front' )
		&&
		// See line in WP_Query::parse_query() at <https://github.com/WordPress/wordpress-develop/blob/0baa8ae/src/wp-includes/class-wp-query.php#L961>.
		0 === count( array_diff( array_keys( wp_parse_args( $query->query ) ), array( amp_get_slug(), 'preview', 'page', 'paged', 'cpage' ) ) )
	);
	if ( $is_front_page_query ) {
		$query->is_home     = false;
		$query->is_page     = true;
		$query->is_singular = true;
		$query->set( 'page_id', get_option( 'page_on_front' ) );
	}
}

/**
 * Whether this is in 'canonical mode'.
 *
 * Themes can register support for this with `add_theme_support( AMP_Theme_Support::SLUG )`:
 *
 *      add_theme_support( AMP_Theme_Support::SLUG );
 *
 * This will serve templates in AMP-first, allowing you to use AMP components in your theme templates.
 * If you want to make available in transitional mode, where templates are served in AMP or non-AMP documents, do:
 *
 *      add_theme_support( AMP_Theme_Support::SLUG, array(
 *          'paired' => true,
 *      ) );
 *
 * Transitional mode is also implied if you define a template_dir:
 *
 *      add_theme_support( AMP_Theme_Support::SLUG, array(
 *          'template_dir' => 'amp',
 *      ) );
 *
 * If you want to have AMP-specific templates in addition to serving AMP-first, do:
 *
 *      add_theme_support( AMP_Theme_Support::SLUG, array(
 *          'paired'       => false,
 *          'template_dir' => 'amp',
 *      ) );
 *
 * If you want to force AMP to always be served on a given template, you can use the templates_supported arg,
 * for example to always serve the Category template in AMP:
 *
 *      add_theme_support( AMP_Theme_Support::SLUG, array(
 *          'templates_supported' => array(
 *              'is_category' => true,
 *          ),
 *      ) );
 *
 * Or if you want to force AMP to be used on all templates:
 *
 *      add_theme_support( AMP_Theme_Support::SLUG, array(
 *          'templates_supported' => 'all',
 *      ) );
 *
 * @see AMP_Theme_Support::read_theme_support()
 * @return boolean Whether this is in AMP 'canonical' mode, that is whether it is AMP-first and there is not a separate (paired) AMP URL.
 */
function amp_is_canonical() {
	if ( ! current_theme_supports( AMP_Theme_Support::SLUG ) ) {
		return false;
	}

	$args = AMP_Theme_Support::get_theme_support_args();
	if ( isset( $args[ AMP_Theme_Support::PAIRED_FLAG ] ) ) {
		return empty( $args[ AMP_Theme_Support::PAIRED_FLAG ] );
	}

	// If there is a template_dir, then transitional mode is implied.
	return empty( $args['template_dir'] );
}

/**
 * Load classes.
 *
 * @since 0.2
 * @deprecated As of 0.6 since autoloading is now employed.
 */
function amp_load_classes() {
	_deprecated_function( __FUNCTION__, '0.6' );
}

/**
 * Add frontend actions.
 *
 * @since 0.2
 */
function amp_add_frontend_actions() {
	add_action( 'wp_head', 'amp_add_amphtml_link' );
}

/**
 * Add post template actions.
 *
 * @since 0.2
 * @deprecated This function is not used when 'amp' theme support is added.
 */
function amp_add_post_template_actions() {
	require_once AMP__DIR__ . '/includes/amp-post-template-functions.php';
	amp_post_template_init_hooks();
}

/**
 * Add action to do post template rendering at template_redirect action.
 *
 * @since 0.2
 * @since 1.0 The amp_render() function is called at template_redirect action priority 11 instead of priority 10.
 * @deprecated This function is not used when 'amp' theme support is added.
 */
function amp_prepare_render() {
	add_action( 'template_redirect', 'amp_render', 11 );
}

/**
 * Render AMP for queried post.
 *
 * @since 0.1
 * @deprecated This function is not used when 'amp' theme support is added.
 */
function amp_render() {
	// Note that queried object is used instead of the ID so that the_preview for the queried post can apply.
	$post = get_queried_object();
	if ( $post instanceof WP_Post ) {
		amp_render_post( $post );
		exit;
	}
}

/**
 * Render AMP post template.
 *
 * @since 0.5
 * @deprecated This function is not used when 'amp' theme support is added.
 *
 * @param WP_Post|int $post Post.
 * @global WP_Query $wp_query
 */
function amp_render_post( $post ) {
	global $wp_query;

	if ( ! ( $post instanceof WP_Post ) ) {
		$post = get_post( $post );
		if ( ! $post ) {
			return;
		}
	}
	$post_id = $post->ID;

	/*
	 * If amp_render_post is called directly outside of the standard endpoint, is_amp_endpoint() will return false,
	 * which is not ideal for any code that expects to run in an AMP context.
	 * Let's force the value to be true while we render AMP.
	 */
	$was_set = isset( $wp_query->query_vars[ amp_get_slug() ] );
	if ( ! $was_set ) {
		$wp_query->query_vars[ amp_get_slug() ] = true;
	}

	// Prevent New Relic from causing invalid AMP responses due the NREUM script it injects after the meta charset.
	if ( extension_loaded( 'newrelic' ) ) {
		newrelic_disable_autorum();
	}

	/**
	 * Fires before rendering a post in AMP.
	 *
	 * This action is not triggered when 'amp' theme support is present. Instead, you should use 'template_redirect' action and check if `is_amp_endpoint()`.
	 *
	 * @since 0.2
	 *
	 * @param int $post_id Post ID.
	 */
	do_action( 'pre_amp_render_post', $post_id );

	amp_add_post_template_actions();
	$template = new AMP_Post_Template( $post );
	$template->load();

	if ( ! $was_set ) {
		unset( $wp_query->query_vars[ amp_get_slug() ] );
	}
}

/**
 * Bootstraps the AMP customizer.
 *
 * Uses the priority of 12 for the 'after_setup_theme' action.
 * Many themes run `add_theme_support()` on the 'after_setup_theme' hook, at the default priority of 10.
 * And that function's documentation suggests adding it to that action.
 * So this enables themes to `add_theme_support( AMP_Theme_Support::SLUG )`.
 * And `amp_init_customizer()` will be able to recognize theme support by calling `amp_is_canonical()`.
 *
 * @since 0.4
 */
function _amp_bootstrap_customizer() {
	add_action( 'after_setup_theme', 'amp_init_customizer', 12 );
}
add_action( 'plugins_loaded', '_amp_bootstrap_customizer', 9 ); // Should be hooked before priority 10 on 'plugins_loaded' to properly unhook core panels.

/**
 * Redirects the old AMP URL to the new AMP URL.
 *
 * If post slug is updated the amp page with old post slug will be redirected to the updated url.
 *
 * @since 0.5
 * @deprecated This function is irrelevant when 'amp' theme support is added.
 *
 * @param string $link New URL of the post.
 * @return string URL to be redirected.
 */
function amp_redirect_old_slug_to_new_url( $link ) {

	if ( is_amp_endpoint() && ! amp_is_canonical() ) {
		if ( current_theme_supports( AMP_Theme_Support::SLUG ) ) {
			$link = add_query_arg( amp_get_slug(), '', $link );
		} else {
			$link = trailingslashit( trailingslashit( $link ) . amp_get_slug() );
		}
	}

	return $link;
}