wp_nav_menu(
    array (
        'theme_location' => 'primary',
        'menu_class' => 'nav-menu',
        'menu_id' => 'header_menu',
        'walker'         => new Sublevel_Walker_m
    )
);

class Sublevel_Walker_m extends Walker_Nav_Menu
{
    function start_lvl( &$output, $depth = 0, $args = array() ) {
        $indent = str_repeat("\t", $depth);
        $output .= "\n$indent<div class='sub-menu'><ul class='sub-menu-ul'>\n";
    }
    function end_lvl( &$output, $depth = 0, $args = array() ) {
        $indent = str_repeat("\t", $depth);
        $output .= "$indent</ul></div>\n";
    }
    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;

      //Getting post id of current page
      $wdm_postid    = url_to_postid( $item->url );

      //$have_children var keeps record of the children of the current page
      $have_children    = false;

      //wdm_has_children() function is a function I've written in functions.php, that checks the children of a post from its post id.
      /*if ( wdm_has_children( $wdm_postid ) ) {
         $have_children = true;
      }else {
       $classes[] = 'menu-item-' . $item->ID;
      }*/

      $class_names    = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args, $depth ) );
      $class_names    = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';
      $id    = apply_filters( 'nav_menu_item_id', 'menu-item-' . $item->ID, $item, $args, $depth );
      $id    = $id ? ' id="' . esc_attr( $id ) . '"' : '';
      $output .= $indent . '<li' . $id . $class_names . '>';
      $atts            = array();
      $atts[ 'title' ]    = ! empty( $item->attr_title ) ? $item->attr_title : '';
      $atts[ 'href' ]    = $item->url;
      $atts[ 'target' ]    = ! empty( $item->target ) ? $item->target : '';
      $atts[ 'rel' ]        = ! empty( $item->xfn ) ? $item->xfn : '';
      $page_name = apply_filters( 'the_title', $item->title, $item->ID );
      $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 . '"';
         }
      }
      //Bootstrap icons - right arrow and down arrow.

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

    public function end_el( &$output, $item, $depth = 0, $args = array() ) {
      $output .= "</li>\n";
     }


}