<?php
/**
 * The separate sub-menu
 *
 * @author Oleg Murashov
 */
function corpobox_get_submenu($args) {
    $defaults = array(
        'container' => 'nav',
        'container_id' => '',
        'container_class' => 'sub-menu-container',
        'submenu_class' => 'sub-menu',
        'submenu_id' => '',
        'xpath' => "./li[contains(@class,'current-menu-item') or contains(@class,'current-menu-ancestor')]/ul",
        'theme_location' => '',
        'echo' => true
    );

    $args = wp_parse_args( $args, $defaults );
    $args = (object) $args;
 
    $menu = wp_nav_menu(
        array(
            'theme_location' => $args->theme_location,
            'container' => '',
            'echo' => false
        )
    );

    $menu = simplexml_load_string($menu);

    $submenu = $menu->xpath($args->xpath);

    if (empty($submenu)) {
        return;
    }

    // Set value of class attribute
    $submenu[0]['class'] = $args->submenu_class;

    // Add "id" attribute
    if ($args->submenu_id) {
        $submenu[0]->addAttribute('id', $args->submenu_id);
    }

    if ($args->container) {
        $submenu_sxe = simplexml_load_string($submenu[0]->saveXML());
        $sdm = dom_import_simplexml($submenu_sxe);

        if ($sdm) {
            $xmlDoc = new DOMDocument('1.0', 'utf-8');
            $container = $xmlDoc->createElement($args->container);

            // Add "class" attribute for container
            if ($args->container_class) {
                $container->setAttribute('class', $args->container_class);
            }

            // Add "id" attribute for container
            if ($args->container_id) {
                $container->setAttribute('id', $args->container_id);
            }
    
            $smsx = $xmlDoc->importNode($sdm, true);
            $container->appendChild($smsx);
            $xmlDoc->appendChild($container);
        }
    }

    if (isset($xmlDoc)) {
        $output = $xmlDoc->saveXML();
    } else {
        $output = $submenu[0]->saveXML();
    }

    if (!$args->echo) {
        return $output;
    }

    echo $output;
}