Mark (highlight) custom post type parent as active item in Wordpress Navigation.When you visit a custom post type's single page, the parent menu item (the post type archive) isn't marked as active. This code solves it by comparing the slug of the current post type with the navigation items, and adds a class accordingly.

Mark parent navigation active when on custom post type single page

<?php

  add_action('nav_menu_css_class', 'add_current_nav_class', 10, 2 );
	
	function add_current_nav_class($classes, $item) {
		
		// Getting the current post details
		global $post;
		
		// Getting the post type of the current post
		$current_post_type = get_post_type_object(get_post_type($post->ID));
		$current_post_type_slug = $current_post_type->rewrite['slug'];
			
		// Getting the URL of the menu item
		$menu_slug = strtolower(trim($item->url));
		
		// If the menu item URL contains the current post types slug add the current-menu-item class
		if (strpos($menu_slug,$current_post_type_slug) !== false) {
		
		   $classes[] = 'active';
		
		}
		
		// Return the corrected set of classes to be added to the menu item
		return $classes;
	
	}

?>

Updated to fix a bug where $current_post_type->rewrite['slug']; was missing quotes in 'slug'. And returned class 'active' for Bootstrap/Underscore based themes.