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    <?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[] = 'current-menu-item';

            }

            // Return the corrected set of classes to be added to the menu item
            return $classes;

        }

    ?>