Use: get_menu('<string : menu_name>') to get nested array or print_menu('<string : menu_name>') to print it.

get_nested_menu_wp.php

<?php
	function get_menu($menu, $items = null, $id = 0) {
		if (is_null($items) && $id == 0) {
			$items = array();
			$menu = wp_get_nav_menu_items($menu);
	
			foreach ($menu as $menu_item) {
				$items[] = array(
					'id'		=> $menu_item->ID,
					'title' 	=> $menu_item->title,
					'url'		=> $menu_item->url,
					'parent'	=> $menu_item->menu_item_parent
				);
			}
		}
	
		$result = array();
		foreach ($items as $index => $item) {
			if ($item['parent'] == $id) {
				$item['subitems'] = get_menu('', $items, $item['id']);
				$result[] = $item;
			}
		}
	
		return $result;
	}
?>

menu_basic_styles.css

.menu.level-0 {
	display: block;
	vertical-align: top;
	list-style: none;
	width: auto;
	height: auto;
}

.menu, .menu ul, .menu li {
	margin: 0; 
	padding: 0;
	box-sizing: border-box;
}

.menu .subitem {
	display: inline-block;
	vertical-align: top;
	position: relative;
	width: auto;
	height: auto;
}

.menu .subitem a {
	display: block;
	vertical-align: top;
	width: auto;
	height: auto;
	padding: 10px 20px;
	white-space: nowrap;
}

.menu .subitem .menu.level-1 .subitem {min-width: 100%;}

.menu .subitem:hover > .menu {
	opacity: 1 !important;
	visibility: visible !important;
}

.menu .subitem .menu[class*="level-"] {
	position: absolute;
	top: 0; left: 100%;
	display: block;
	vertical-align: top;
	list-style: none;
	width: auto;
	height: auto;
	opacity: 0;
	visibility: hidden;
	transition: all .2s ease-in-out;
}

.menu .subitem .menu.level-1 {top: 100%; left: 0;}

@media (max-width: 700px) {
	.menu .subitem {
		display: block;
	}

	.menu .subitem .menu[class*="level-"] {
		position: relative !important; 
		left: 0 !important;
		height: 0 !important;
	}

	.menu .subitem:hover > .menu[class*="level-"] {
		height: auto !important;
	}
}

print_nested_menu_wp.php

<?php
	function print_menu($menu, $items = null, $level = 0) {
		$items = (is_null($items)) ? get_menu($menu) : $items;
		$html = "<ul class='menu level-". $level ."'>";
		foreach ($items as $item) {
			$html .= "<li class='subitem'>";
			$html .= "<a href='". $item['url'] ."'>". $item['title'] ."</a>";
	
			if ($item['subitems']) 
				$html .= print_menu(null, $item['subitems'], $level + 1);
	
			$html .= "</li>";
		}
		$html .= "</ul>";
		return $html;
	}
?>