Add custom ID attribute to all WordPress submenus. Great for building mega menus.

davey-jacobson-custom-nav-walker.php

<?php
/**
 * Nav Menu API: DJ_NavWalker class.
 *
 * I cloned & modified this class to help improve compatibility for
 * WordPress and the Foundation 5.5.3 `dropdown` module.
 *
 * @author Davey Jacobson <djacobson@usarugby.org>
 *
 * @package Davey_Jacobson
 * @subpackage NavWalker
 * @since 2.5.0
 */

/**
 * Core class used to implement an HTML list of nav menu items.
 *
 * @since 3.0.0
 *
 * @see Walker
 */
class DJ_NavWalker extends Walker_Nav_Menu {

    /**
     * Change this according to your website's main menu slug.
     *
     * @access private
     * @var string
     */
    private static $main_menu = 'main-menu';
    
    /**
     * Custom postfix value to be appended to the `ID`.
     *
     * @access private
     * @var string
     */
    private static $postfix = '-menu';

    /**
     * The main sub-menu list container.
     *
     * @access private
     * @var array
     */
    private static $sub_menus = array();

    /**
     * Items to be renamed; adjust according to your website.
     *
     * @access private
     * @var array {
     *
     *    @type string `$item->ID` => @type string `$custom_slug`
     *
     *}
     */
    private static $renamed = array(
        //'21464' => 'about'
    );

    /**
     * The most important piece of this whole puzzle.
     *
     * @access private
     * @var int
     */
    private static $index = -1;

    /**
     * Starts the list before the elements are added.
     *
     * @since 3.0.0
     *
     * @see Walker::start_lvl()
     *
     * @param string   $output Used to append additional content (passed by reference).
     * @param int      $depth  Depth of menu item. Used for padding.
     * @param stdClass $args   An object of wp_nav_menu() arguments.
     */
    public function start_lvl( &$output, $depth = 0, $args = array() ) {
        self::$index++;

        if ( isset( $args->item_spacing ) && 'discard' === $args->item_spacing ) {
            $t = '';
            $n = '';
        } else {
            $t = "\t";
            $n = "\n";
        }
        $indent = str_repeat( $t, $depth );

        // Default class.
        $classes = array( 'sub-menu' );

        /**
         * Filters the CSS class(es) applied to a menu list element.
         *
         * @since 4.8.0
         *
         * @param string[] $classes Array of the CSS classes that are applied to the menu `<ul>` element.
         * @param stdClass $args    An object of `wp_nav_menu()` arguments.
         * @param int      $depth   Depth of menu item. Used for padding.
         */
        $class_names = join( ' ', apply_filters( 'nav_menu_submenu_css_class', $classes, $args, $depth ) );
        $class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';

        /**
         * Add a specified `ID` attribute to all main menu dropdowns.
         *
         * @link {@see 'wp_get_nav_menu_items'}
         */
        if ( self::$main_menu === $args->menu->slug ) { /** MAKE SURE YOU RENAME THE `$main_menu` PROPERTY FOR YOUR WEBSITE. */
            /**
             * The final list of available dropdowns.
             *
             * @see self::$sub_menus
             *
             * @var array
             */
            $sub_menus = array();

            /**
             * Grab only `$items` whose parent's object ID is `0`.
             *
             * @var array
             */
            $items = wp_get_nav_menu_items( self::$main_menu );
            $items = array_map( array( $this, 'remove_none_dropdown_menus' ), $items );

            // Ensure there are no duplicates or `null` values.
            array_filter( $items );

            // If we have dropdown menus...
            if ( $items ) {
                foreach ( $items as $i => $item ) {
                    if ( isset( $item->url ) ) {
                        $hash     = wp_parse_url( $item->url, PHP_URL_FRAGMENT );
                        $path     = wp_parse_url( $item->url, PHP_URL_PATH );
                        $_menu    = empty( $path ) ? $hash : ltrim( $path, '/' );
                        $sub_menu = untrailingslashit( $_menu );
                        // In case we've specifed `$items` that need to be renamed...
                        if ( isset( self::$renamed ) && ! empty( self::$renamed ) ) {
                            foreach ( self::$renamed as $id => $slug ) {
                                if ( $id === @$item->ID ) {
                                    $sub_menu = $slug;
                                }
                            }
                        }
                    }
                    // Ensure we only have clean slugs.
                    $sub_menus[] = stripslashes( $sub_menu );
                }
            }

            // Final, cleaned sub-menu list.
            self::$sub_menus = array_values( array_unique( $sub_menus ) );

            // Begin building the final sub-menu output.
            $output .= "{$n}{$indent}<ul{$class_names} id='";
                // The actual `ID` attribute value.
                $output .= esc_attr( @array_slice( self::$sub_menus, self::$index )[0] . self::$postfix );
                // Added compatibility for Foundation 5.5.3.
                $output .= "' data-dropdown-content>{$n}";
        }
        else {
            $output .= "{$n}{$indent}<ul{$class_names}>{$n}";
        }

    }

    /**
     * Starts the element output.
     *
     * @since 3.0.0
     * @since 4.4.0 The {@see 'nav_menu_item_args'} filter was added.
     *
     * @see Walker::start_el()
     *
     * @param string   $output Used to append additional content (passed by reference).
     * @param WP_Post  $item   Menu item data object.
     * @param int      $depth  Depth of menu item. Used for padding.
     * @param stdClass $args   An object of wp_nav_menu() arguments.
     * @param int      $id     Current item ID.
     */
    public function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {

        if ( isset( $args->item_spacing ) && 'discard' === $args->item_spacing ) {
            $t = '';
            $n = '';
        } else {
            $t = "\t";
            $n = "\n";
        }
        $indent = ( $depth ) ? str_repeat( $t, $depth ) : '';

        $classes   = empty( $item->classes ) ? array() : (array) $item->classes;
        $classes[] = 'menu-item-' . $item->ID;

        /**
         * Filters the arguments for a single nav menu item.
         *
         * @since 4.4.0
         *
         * @param stdClass $args  An object of wp_nav_menu() arguments.
         * @param WP_Post  $item  Menu item data object.
         * @param int      $depth Depth of menu item. Used for padding.
         */
        $args = apply_filters( 'nav_menu_item_args', $args, $item, $depth );

        /**
         * Filters the CSS classes applied to a menu item's list item element.
         *
         * @since 3.0.0
         * @since 4.1.0 The `$depth` parameter was added.
         *
         * @param string[] $classes Array of the CSS classes that are applied to the menu item's `<li>` element.
         * @param WP_Post  $item    The current menu item.
         * @param stdClass $args    An object of wp_nav_menu() arguments.
         * @param int      $depth   Depth of menu item. Used for padding.
         */
        $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args, $depth ) );
        $class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';

        /**
         * Filters the ID applied to a menu item's list item element.
         *
         * @since 3.0.1
         * @since 4.1.0 The `$depth` parameter was added.
         *
         * @param string   $menu_id The ID that is applied to the menu item's `<li>` element.
         * @param WP_Post  $item    The current menu item.
         * @param stdClass $args    An object of wp_nav_menu() arguments.
         * @param int      $depth   Depth of menu item. Used for padding.
         */
        $id = apply_filters( 'nav_menu_item_id', 'menu-item-' . $item->ID, $item, $args, $depth );
        $id = $id ? ' id="' . esc_attr( $id ) . '"' : '';
        
        /**
         * Add a custom `HTML5` attribute to list item tag if linked directly any `post_type`.
         *
         * @var string
         */
        if ( '#' !== $item->url ) {
            $url_path = wp_parse_url( $item->url, PHP_URL_PATH );
            // Get the actual WP_Post object.
            $post = get_page_by_path( $url_path );
            // Default to `post_id` of 0 if `$post` not found.
            $post_id = !is_null( $post ) ? $post->ID : 0;
        }
        // Custom HTML5 data attribute on list item tag.
        $dataID = ' data-id="' . ( 0 !== $post_id ? esc_attr( $post_id ) : 'external' ) . '"';
        // Generate the output.
        $output .= "{$indent}<li{$id}{$class_names}{$dataID}>";

        $atts           = array();
        $atts['title']  = ! empty( $item->attr_title ) ? $item->attr_title : '';
        $atts['target'] = ! empty( $item->target ) ? $item->target : '';
        // Automatic handling of URL targets and relationships (for SEO).
        if ( '_blank' === $item->target && empty( $item->xfn ) ) {
            $atts['rel'] = 'noopener noreferrer';
        } else {
            $atts['rel'] = $item->xfn;
        }
        $atts['href']         = ! empty( $item->url ) ? $item->url : '';
        $atts['aria-current'] = $item->current ? 'page' : '';

        /**
         * Filters the HTML attributes applied to a menu item's anchor element.
         *
         * @since 3.6.0
         * @since 4.1.0 The `$depth` parameter was added.
         *
         * @param array $atts {
         *     The HTML attributes applied to the menu item's `<a>` element, empty strings are ignored.
         *
         *     @type string $title        Title attribute.
         *     @type string $target       Target attribute.
         *     @type string $rel          The rel attribute.
         *     @type string $href         The href attribute.
         *     @type string $aria_current The aria-current attribute.
         * }
         * @param WP_Post  $item  The current menu item.
         * @param stdClass $args  An object of wp_nav_menu() arguments.
         * @param int      $depth Depth of menu item. Used for padding.
         */
        $atts = apply_filters( 'nav_menu_link_attributes', $atts, $item, $args, $depth );

        $attributes = '';
        foreach ( (array) $atts as $attr => $value ) {
            if ( ! empty( $value ) ) {
                $value       = ( 'href' === $attr ) ? esc_url( $value ) : esc_attr( $value );
                $attributes .= ' ' . $attr . '="' . $value . '"';
            }
        }

        /** This filter is documented in wp-includes/post-template.php */
        $title = apply_filters( 'the_title', $item->title, $item->ID );

        /**
         * Filters a menu item's title.
         *
         * @since 4.4.0
         *
         * @param string   $title The menu item's title.
         * @param WP_Post  $item  The current menu item.
         * @param stdClass $args  An object of wp_nav_menu() arguments.
         * @param int      $depth Depth of menu item. Used for padding.
         */
        $title = apply_filters( 'nav_menu_item_title', $title, $item, $args, $depth );

        $item_output  = $args->before;
        $item_output .= '<a' . $attributes . '>';
        $item_output .= $args->link_before . $title . $args->link_after;
        $item_output .= '</a>';
        $item_output .= $args->after;

        /**
         * Filters a menu item's starting output.
         *
         * The menu item's starting output only includes `$args->before`, the opening `<a>`,
         * the menu item's title, the closing `</a>`, and `$args->after`. Currently, there is
         * no filter for modifying the opening and closing `<li>` for a menu item.
         *
         * @since 3.0.0
         *
         * @param string   $item_output The menu item's starting HTML output.
         * @param WP_Post  $item        Menu item data object.
         * @param int      $depth       Depth of menu item. Used for padding.
         * @param stdClass $args        An object of wp_nav_menu() arguments.
         */
        $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );

    }

    /**
     * Ensure we're only retrieving the dropdown menus.
     *
     * @access private
     *
     * @return object    Dropdown menus.
     */
    private function remove_non_dropdown_menus( $item ) {
        if ( '0' === $item->menu_item_parent ) {
            return $item;
        }
    }

}