bjornjohansen/wordpress-menu-cache.php

## wordpress-menu-cache.php
<?php
/**
 * WordPress menu cache.
 *
 * @package BJ\Menu
 * @author bjornjohansen
 * @version 0.1.0
 * @license https://www.gnu.org/licenses/old-licenses/gpl-2.0.html  GNU General Public License version 2 (GPLv2)
 */

/**
 * Short-circuit the wp_nav_menu() output if we have cached output ready.
 *
 * Returning a non-null value to the filter will short-circuit
 * wp_nav_menu(), echoing that value if $args->echo is true,
 * returning that value otherwise.
 *
 * @see wp_nav_menu()
 *
 * @param string|null $output Nav menu output to short-circuit with. Default null.
 * @param stdClass    $args   An object containing wp_nav_menu() arguments.
 * @return string|null Nav menu output to short-circuit with. Passthrough (default null) if we don’t have a cached version.
 */
add_filter( 'pre_wp_nav_menu', function( $output, $args ) {

	/* This section is from wp_nav_menu(). It is here to find a menu when none is provided. */

	// @codingStandardsIgnoreStart

	// Get the nav menu based on the requested menu
	$menu = wp_get_nav_menu_object( $args->menu );

	// Get the nav menu based on the theme_location
	if ( ! $menu && $args->theme_location && ( $locations = get_nav_menu_locations() ) && isset( $locations[ $args->theme_location ] ) )
		$menu = wp_get_nav_menu_object( $locations[ $args->theme_location ] );

	// get the first menu that has items if we still can't find a menu
	if ( ! $menu && !$args->theme_location ) {
		$menus = wp_get_nav_menus();
		foreach ( $menus as $menu_maybe ) {
			if ( $menu_items = wp_get_nav_menu_items( $menu_maybe->term_id, array( 'update_post_term_cache' => false ) ) ) {
				$menu = $menu_maybe;
				break;
			}
		}
	}

	if ( empty( $args->menu ) ) {
		$args->menu = $menu;
	}
	// @codingStandardsIgnoreEnd
	/* End of the section from wp_nav_menu(). It was a pleasure, ladies and gents. */

	global $wp_query;
	$menu_signature = md5( wp_json_encode( $args ) . $wp_query->query_vars_hash );

	// We don’t actually need the references to all the cached versions of this menu,
	// but we need to make sure the cache is not out of sync - transients are unreliable.
	$cached_versions = get_transient( 'menu-cache-menuid-' . $args->menu->term_id );
	if ( false !== $cached_versions ) {
		$cached_output = get_transient( 'menu-cache-' . $menu_signature );
		if ( false !== $cached_output ) {
			$output = $cached_output;
		}
	}

	return $output;
}, 10, 2 );

/**
 * Cache the HTML content output for navigation menus.
 *
 * @see wp_nav_menu()
 *
 * @param string   $nav_menu The HTML content for the navigation menu.
 * @param stdClass $args     An object containing wp_nav_menu() arguments.
 * @return string The HTML content for the navigation menu.
 */
add_filter( 'wp_nav_menu', function( $nav_menu, $args ) {
	global $wp_query;
	$menu_signature = md5( wp_json_encode( $args ) . $wp_query->query_vars_hash );

	if ( isset( $args->menu->term_id ) ) {
		set_transient( 'menu-cache-' . $menu_signature, $nav_menu );

		// Store a reference to this version of the menu, so we can purge it when needed.
		$cached_versions = get_transient( 'menu-cache-menuid-' . $args->menu->term_id );
		if ( false === $cached_versions ) {
			$cached_versions = [];
		} else {
			$cached_versions = json_decode( $cached_versions );
		}
		if ( ! in_array( $menu_signature, $cached_versions, true ) ) {
			$cached_versions[] = $menu_signature;
		}
		set_transient( 'menu-cache-menuid-' . $args->menu->term_id, wp_json_encode( $cached_versions ), 15552000 );
	}

	return $nav_menu;
}, 10, 2 );

/**
 * Clears the menu cache.
 *
 * Fires after a navigation menu has been successfully updated.
 *
 * @param int   $menu_id   ID of the updated menu.
 * @param array $menu_data An array of menu data.
 */
add_action( 'wp_update_nav_menu', function( $menu_id, $menu_data = null ) {
	if ( is_array( $menu_data ) && isset( $menu_data['menu-name'] ) ) {
		$menu = wp_get_nav_menu_object( $menu_data['menu-name'] );
		if ( isset( $menu->term_id ) ) {
			// Get all cached versions of this menu and delete them.
			$cached_versions = get_transient( 'menu-cache-menuid-' . $menu->term_id );
			if ( false !== $cached_versions ) {
				$cached_versions = json_decode( $cached_versions );
				foreach ( $cached_versions as $menu_signature ) {
					delete_transient( 'menu-cache-' . $menu_signature );
				}
				set_transient( 'menu-cache-menuid-' . $menu->term_id, wp_json_encode( [] ), 15552000 );
			}
		}
	}
}, 10, 2 );