Generate multilevel nested menu in Wordpress using recursion function

multilevel_menu_wp_recursion.php

<?php
/**
 * Custom menu generator
 */
function generateTreeNavwalker(array &$menuItems, $parentId = 0) {
	$branch = array();
	foreach ($menuItems as &$item) {
		if ($item->menu_item_parent == $parentId) {
			$children = generateTreeNavwalker($menuItems, $item->ID);
			if ($children) {
				if (!$item->submenu_items) {
					$item->submenu_items = array();
				}
				$item->submenu_items = $children;
			}
			$branch[$item->ID] = $item;
			unset($menuItems[$item->ID]);
		}
	}

	return $branch;
}

/**
 * Output Custom Menu
 */
function outputCustomMenuNavwalker(array &$menuItems, $parentId = 0) {
  foreach ($menuItems as &$item) {
    if ($item->menu_item_parent == $parentId) {
			$menuHtml .= '<li class="nav-item menu-item item-'.$item->object_id.'">
				<a class="nav-link" href="'.$item->url.'">'.$item->title.'</a>';
				$subMenu = outputCustomMenuNavwalker($menuItems, $item->ID);
				if ($subMenu) {
					$menuHtml .= '<ul class="dropdown-menu">';
						$menuHtml .= $subMenu;
					$menuHtml .= '</ul>';
				}
			$menuHtml .= '</li>';
		}
  }
	
  return $menuHtml;
}