menu_loader_exploder.php

<?php
$locations  = get_nav_menu_locations();
$menu       = wp_get_nav_menu_object( $locations['topnav'] );
$menu_items = wp_get_nav_menu_items( $menu->term_id, array(
	'order' => 'DESC',
) );
?>
		
<select class="the-selectors" onChange="document.location.href=this.options[this.selectedIndex].value;">
	<option value="">
		<?php _e( 'Please Select', 'your_textdomain' ); ?>
	</option>
	<?php
	$i = 0;
	$html = '';
	foreach ( $menu_items as &$item )
	{
		if ( 0 === $item->menu_item_parent )
		{
			$item->sub_menu = 0;
		}
		// the last node was my parent. I am first born, I shall gain a level now.
		elseif( 
			isset( $menu_items[ $i -1 ] )
			AND $menu_items[ $i -1 ]->ID === $item->menu_item_parent
			)
		{
			$item->sub_menu = $menu_items[ $i -1 ]->sub_menu +1;
			
		}
		// The last node is my sibling. We share a parent.
		// I shall reference my sibling's sub_menu:
		elseif(
			isset( $menu_items[ $i -1 ] )
			AND $menu_items[ $i -1 ]->menu_item_parent === $item->menu_item_parent
			)
		{
			
			$item->sub_menu = $menu_items[ $i -1 ]->sub_menu;
			
		}
		// I have a parent, but it's not same as last node.
		// I shall reference my last known sibling:
		else
		{
			for( $a = $i -1; $a >= 0; $a-- )
			{
				if ( 
					isset( $menu_items[ $a ] )
					AND $menu_items[ $a ]->menu_item_parent === $item->menu_item_parent
					)
				{
					$item->sub_menu = $menu_items[ $a ]->sub_menu;
					break;
				}
			}
		}
		
		$html .= sprintf(
			'<option value="%s">%s%s%s</option>',
			$item->url,
			str_repeat( '-&nbsp;', count( $item->submenu ) -1 ),
			$item->title,
			'#' === $item->url ? '&raquo;' : ''
		);
		
		$i++;
	}
	echo $html;
	unset( $locations, $menu, $menu_items, $html, $i, $a );
	?>
</select>