WordPress - Create in-section navigation for a given (hierarchical) post type

gistfile1.php

<?php

// Return link by post ID
// -------------------------------------------------------------
function link_by_id($post_id) {
	echo '<a href="'.get_permalink($post_id).'">'.get_the_title($post_id).'</a>';
}

// Return the root parent ID
// -------------------------------------------------------------
function get_root_parent_id($post_id = null) {
	if(!$post_id) {
		global $post;
		$post_id = $post->ID;
	}
	global $wpdb;
	$parent = $wpdb->get_var("SELECT post_parent FROM $wpdb->posts WHERE post_type='page' AND post_status='publish' AND ID = '$page_id'");
	if ($parent == 0) return $post_id;
	else return root_parent_id($parent);
}

// Return root posts of any given post type
// -------------------------------------------------------------
function get_root_pages($post_type = 'page') {
	$args = array(
		'post_type' => $post_type,
		'parent' => 0,
		'order' => 'ASC',
		'orderby' => 'menu_order',
		'numberposts' => -1
	);	
	return get_pages($args);
}

// Return sub pages of a post
// -------------------------------------------------------------
function get_sub_pages($post_id, $post_type = 'page') {
	$args = array(
		'post_type' => $post_type,
		'child_of' => $post_id,
		'parent' => $post_id,
		'sort_order' => 'ASC',
		'sort_column' => 'menu_order'
	);	
	return get_pages($args);
}

// PLUGIN - Generate uber sub-navigation (in this section)
// -------------------------------------------------------------
// boons_insection_subnav('product', 'current');
// Usage: 'product' = post type and 'current' = current class for <li>
// -------------------------------------------------------------
function boons_insection_subnav($post_type = 'page', $current_class = 'current') {

	// Test if current page in sub navigation
	// -------------------------------------------------------------
	function subnav_is_current($post_id) {
		global $post;
		return ($post_id==$post->ID);
	}

	// Test next level in sub navigation
	// -------------------------------------------------------------
	function subnav_next_level($post_id, $next_level) {
		global $post;
		return ($next_level && subnav_is_current($post_id) || in_array($post_id, $post->ancestors));
	}

	// Output subnav
	// -------------------------------------------------------------
	// level 1
	$sub_level_1 = get_root_pages($post_type);
	if($sub_level_1): ?>
	<ul class="navigation-secondary level-1">
		<?php
		// level 1 items
		foreach($sub_level_1 as $level_1_page):
		$level_1_page_id = $level_1_page->ID;
		?>
		<li class="level-1-item<?= subnav_is_current($level_1_page_id) ? ' '.$current_class.'' : ''; ?>">
			<?php link_by_id($level_1_page_id); ?>
			<?php
			// level 2
			$sub_level_2 = get_sub_pages($level_1_page_id, $post_type);
			if(subnav_next_level($level_1_page_id, $sub_level_2)): ?>
			<ul class="level-2">
				<?php
				// level 2 items
				foreach($sub_level_2 as $level_2_page):
				$level_2_page_id = $level_2_page->ID;
				?>
				<li class="level-2-item<?= subnav_is_current($level_2_page_id) ? ' '.$current_class.'' : ''; ?>">
					<?php link_by_id($level_2_page_id); ?>
					<?php
					// level 3
					$sub_level_3 = get_sub_pages($level_2_page_id, $post_type);
					if(subnav_next_level($level_2_page_id, $sub_level_3)): ?>
					<ul class="level-3">
						<?php
						// level 3 items
						foreach($sub_level_3 as $level_3_page):
						$level_3_page_id = $level_3_page->ID;
						?>
						<li class="level-3-item<?= subnav_is_current($level_3_page_id) ? ' '.$current_class.'' : ''; ?>">
							<?php link_by_id($level_3_page_id); ?>
						</li>
						<?php
						// level 3 items
						endforeach; ?>
					</ul>
					<?php
					// level 3
					endif; ?>
				</li>
				<?php
				// level 2 items
				endforeach; ?>
			</ul>
			<?php
			// level 2
			endif; ?>
		</li>
		<?php
		// level 1 items
		endforeach; ?>
	</ul>
	<?php else: ?>
	<em>No <?= $post_type; ?>s added yet</em>
	<?php endif; ?>
<?
}
?>