Dropdown BS Nav

header.php

<?php require_once(get_template_directory() . '/lib/nav.php'); ?>
<header class="banner navbar navbar-default navbar-static-top" role="banner">
  <div class="container">
    <div class="navbar-header">
      <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target=".navbar-collapse">
        <span class="sr-only"><?= __('Toggle navigation', 'sage'); ?></span>
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
      </button>
      <a class="navbar-brand" href="<?= esc_url(home_url('/')); ?>"><?php bloginfo('name'); ?></a>
    </div>

    <nav class="collapse navbar-collapse" role="navigation">
      <?php
      if (has_nav_menu('primary_navigation')) :

        //wp_nav_menu(['theme_location' => 'primary_navigation', 'walker' => new Roots\Soil\Nav\NavWalker(), 'menu_class' => 'nav navbar-nav']);
        wp_nav_menu(['theme_location' => 'primary_navigation', 'walker' => new Roots\Sage\Nav\NavWalker(), 'menu_class' => 'nav navbar-nav']);

      endif;

      //wp_bootstrap_navwalker
      ?>
    </nav>
  </div>
</header>

nav.php

<?php
namespace Roots\Sage\Nav;

/**
 * Cleaner walker for wp_nav_menu() - for sage theme
 * 
 * @see https://github.com/roots/sage
 * @see https://github.com/roots/soil
 * 
 * Soil handles clean menus with BS markup no probs, but doesn't handle dropdowns.
 * This does - taken from the old Roots /lib/nav.php and adapted for namespacing.
 * Drop this file into lib or somewhere logical, and include it in header.php. Then use
 * Bootstrap markup for the menu like the accompanying file.
 *
 * Walker_Nav_Menu (WordPress default) example output:
 *   <li id="menu-item-8" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-8"><a href="/">Home</a></li>
 *   <li id="menu-item-9" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-9"><a href="/sample-page/">Sample Page</a></l
 *
 * Roots_Nav_Walker example output:
 *   <li class="menu-home"><a href="/">Home</a></li>
 *   <li class="menu-sample-page"><a href="/sample-page/">Sample Page</a></li>
 */
class NavWalker extends \Walker_Nav_Menu {

  function check_current( $classes ) {

    return preg_match('/(current[-_])|active|dropdown/', $classes);

  }

  function start_lvl( &$output, $depth = 0, $args = array() ) {

    $output .= "\n<ul class=\"dropdown-menu\">\n";

  }

  function start_el(&$output, $item, $depth = 0, $args = array(), $id = 0) {

    $item_html = '';
    parent::start_el($item_html, $item, $depth, $args);

    if ( $item->is_dropdown && ( $depth === 0 ) ) {

      $item_html = str_replace('<a', '<a class="dropdown-toggle" data-toggle="dropdown" data-target="#"', $item_html);
      $item_html = str_replace('</a>', ' <b class="caret"></b></a>', $item_html);

    } elseif (stristr($item_html, 'li class="divider')) {

      $item_html = preg_replace('/<a[^>]*>.*?<\/a>/iU', '', $item_html);

    } elseif (stristr($item_html, 'li class="dropdown-header')) {

      $item_html = preg_replace('/<a[^>]*>(.*)<\/a>/iU', '$1', $item_html);

    }

    $item_html = apply_filters('roots/wp_nav_menu_item', $item_html);
    $output .= $item_html;

  }

  function display_element( $element, &$children_elements, $max_depth, $depth = 0, $args, &$output ) {

    $element->is_dropdown = ( ( !empty( $children_elements[$element->ID] ) && ( ( $depth + 1 ) < $max_depth || ( $max_depth === 0 ) ) ) );

    if ( $element->is_dropdown ) {

      $element->classes[] = 'dropdown';

    }

    parent::display_element($element, $children_elements, $max_depth, $depth, $args, $output);

  }

}

/**
 * Remove the id="" on nav menu items
 * Return 'menu-slug' for nav menu classes
 */
function roots_nav_menu_css_class( $classes, $item ) {

  $slug = sanitize_title($item->title);
  $classes = preg_replace('/(current(-menu-|[-_]page[-_])(item|parent|ancestor))/', 'active', $classes);
  $classes = preg_replace('/^((menu|page)[-_\w+]+)+/', '', $classes);

  $classes[] = 'menu-' . $slug;

  $classes = array_unique($classes);

  return array_filter($classes, __NAMESPACE__ . '\\is_element_empty');

}

add_filter( 'nav_menu_css_class', __NAMESPACE__ . '\\roots_nav_menu_css_class', 10, 2 );
add_filter( 'nav_menu_item_id', '__return_null' );

/**
 * Clean up wp_nav_menu_args
 *
 * Remove the container
 * Use Roots_Nav_Walker() by default
 */
function roots_nav_menu_args( $args = '' ) {

  $roots_nav_menu_args = array();

  $roots_nav_menu_args['container'] = false;

  if (!$args['items_wrap']) {
    $roots_nav_menu_args['items_wrap'] = '<ul class="%2$s">%3$s</ul>';
  }

  if (!$args['depth']) {
    $roots_nav_menu_args['depth'] = 2;
  }

  if (!$args['walker']) {
    $roots_nav_menu_args['walker'] = new Nav\NavWalker();
  }

  return array_merge($args, $roots_nav_menu_args);

}

add_filter( 'wp_nav_menu_args', __NAMESPACE__ . '\\roots_nav_menu_args' );

function is_element_empty( $element ) {

  $element = trim($element);
  return !empty($element);

}