Menu Walker for wordpress, that changes the order of items, which have a submenu. The submenu is first (standard is anchor first)

SubmenFirstWalker.php

<?php

/**
 * Class SubmenuFirstWalker
 * This walker changes the order of elements, if they have a submenu
 * Standard is anchor than the submenu. Wth this walker, the submenu gets rendered first.
 * There is one new filter 'walker_nav_menu_end_el'
 * @author Markus Schober
 */
class SubmenuFirstWalker extends Walker_Nav_Menu {

    public function display_element( $element, &$children_elements, $max_depth, $depth, $args, &$output ) {

        if ( ! $element ) {
            return;
        }

        $id_field = $this->db_fields['id'];
        $id       = $element->$id_field;

        //display this element
        $this->has_children = ! empty( $children_elements[ $id ] );
        if ( isset( $args[0] ) && is_array( $args[0] ) ) {
            $args[0]['has_children'] = $this->has_children; // Backwards compatibility.
        }

        $cb_args = array_merge( array( &$output, $element, $depth ), $args );
        call_user_func_array( array( $this, 'start_el' ), $cb_args );

        // descend only when the depth is right and there are childrens for this element
        if ( ( $max_depth == 0 || $max_depth > $depth + 1 ) && isset( $children_elements[ $id ] ) ) {

            foreach ( $children_elements[ $id ] as $child ) {

                if ( ! isset( $newlevel ) ) {
                    $newlevel = true;
                    //start the child delimiter
                    $cb_args = array_merge( array( &$output, $depth ), $args );
                    call_user_func_array( array( $this, 'start_lvl' ), $cb_args );
                }
                $this->display_element( $child, $children_elements, $max_depth, $depth + 1, $args, $output );
            }
            unset( $children_elements[ $id ] );
        }

        if ( isset( $newlevel ) && $newlevel ) {
            //end the child delimiter
            $cb_args = array_merge( array( &$output, $depth ), $args );
            call_user_func_array( array( $this, 'end_lvl' ), $cb_args );
        }

        //end this element
        $cb_args = array_merge( array( &$output, $element, $depth ), $args );
        call_user_func_array( array( $this, 'end_el' ), $cb_args );
    }


    public function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
        $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

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

        /**
         * Filter the CSS class(es) applied to a menu item's list item element.
         *
         * @since 3.0.0
         * @since 4.1.0 The `$depth` parameter was added.
         *
         * @param array  $classes The CSS classes that are applied to the menu item's `<li>` element.
         * @param object $item    The current menu item.
         * @param array  $args    An array of {@see 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 ) . '"' : '';

        /**
         * Filter 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 object $item    The current menu item.
         * @param array  $args    An array of {@see 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 ) . '"' : '';

        $start_el = $indent . '<li' . $id . $class_names . '>';
        $output .= apply_filters( 'walker_nav_menu_start_el', $start_el, $item, $depth, $args );
    }


    public function end_el( &$output, $item, $depth = 0, $args = array() ) {
        $atts           = array();
        $atts['title']  = ! empty( $item->attr_title ) ? $item->attr_title : '';
        $atts['target'] = ! empty( $item->target ) ? $item->target : '';
        $atts['rel']    = ! empty( $item->xfn ) ? $item->xfn : '';
        $atts['href']   = ! empty( $item->url ) ? $item->url : '';

        /**
         * Filter 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.
         * }
         *
         * @param object $item   The current menu item.
         * @param array  $args   An array of {@see 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 ( $atts as $attr => $value ) {
            if ( ! empty( $value ) ) {
                $value = ( 'href' === $attr ) ? esc_url( $value ) : esc_attr( $value );
                $attributes .= ' ' . $attr . '="' . $value . '"';
            }
        }

        $item_output = $args->before;
        $item_output .= '<a' . $attributes . '>';
        /** This filter is documented in wp-includes/post-template.php */
        $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
        $item_output .= '</a>';
        $item_output .= $args->after;

        /**
         * Filter 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 object $item        Menu item data object.
         * @param int    $depth       Depth of menu item. Used for padding.
         * @param array  $args        An array of {@see wp_nav_menu()} arguments.
         */
        $output .= apply_filters( 'walker_nav_menu_end_el', $item_output, $item, $depth, $args );
        $output .= "</li>\n";
    }
}