<?php

add_filter( 'wp_nav_menu_objects', 'baw_nav_menu_objects', 10, 2 );
function baw_nav_menu_objects( $items, $args ) {
    static $duplicate_menus = array();
    if ( isset( $args->theme_location, $duplicate_menus[ $args->theme_location ] ) ||
        isset( $args->menu->term_id) && in_array( $args->menu->term_id, $duplicate_menus ) ) {

        $current = 0;
        $new_items = array();
        foreach ( $items as $item ) {
            if ( ! $item->menu_item_parent && ( $item->current || $item->current_item_ancestor ) ) {
                $current = $item;
                reset( $items );
                foreach ( $items as $item ) {
                    if ( $item->menu_item_parent == $current->ID ) {
                        $new_items[] = $item;
                    }
                }
                break;
            }
        }

        ////array_unshift( $new_items, $current ); // laisser ou pas le menu parent
        if ( $current ) { // vider ou pas le menu
            $items = $new_items;
        }
    }
    if ( ! empty( $args->theme_location ) ) {
        $locations = get_nav_menu_locations();
        $duplicate_menus[ $args->theme_location ] = $locations[ $args->theme_location ];
    }

    return $items;
}