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

   function add_current_nav_class($classes, $item) {

      // Getting the current post details
      global $post;

      // Get post ID, if nothing found set to NULL
      $id = ( isset( $post->ID ) ? get_the_ID() : NULL );

      // Checking if post ID exist...
      if (isset( $id )){

          // Getting the post type of the current post
          $current_post_type = get_post_type_object(get_post_type($post->ID));

          // Getting the rewrite slug containing the post type's ancestors
          $ancestor_slug = $current_post_type->rewrite ? $current_post_type->rewrite['slug'] : '';

          // Split the slug into an array of ancestors and then slice off the direct parent.
          $ancestors = explode('/',$ancestor_slug);
          $parent = array_pop($ancestors);

          // Getting the URL of the menu item
          $menu_slug = strtolower(trim($item->url));

          // Remove domain from menu slug
          $menu_slug = str_replace($_SERVER['SERVER_NAME'], "", $menu_slug);

          // If the menu item URL contains the post type's parent
          if (!empty($menu_slug) && !empty($parent) && strpos($menu_slug,$parent) !== false) {
              $classes[] = 'current-menu-item';
          }

          // If the menu item URL contains any of the post type's ancestors
          foreach ( $ancestors as $ancestor ) {
              if (!empty($menu_slug) && !empty($ancestor) && strpos($menu_slug,$ancestor) !== false) {
                  $classes[] = 'current-page-ancestor';
              }
          }
      }
      // Return the corrected set of classes to be added to the menu item
      return $classes;

   }
   add_action('nav_menu_css_class', 'add_current_nav_class', 10, 2 );

Had same problem as @rebdev solved by changing:

// If the menu item URL contains the post type's parent
        if (strpos($menu_slug,$parent) !== false) {
            $classes[] = 'current-menu-item';
        }

to:

        // If the menu item URL contains the post type's parent
        if (!empty($menu_slug) && !empty($parent) && strpos($menu_slug,$parent) !== false) {
            $classes[] = 'current-menu-item';
        }

Thanks for the fix and thanks to @Kelderic for the code.
Its exactly what i was looking for. It works like a charm.
But i noticed a funny error. I have a slug for a CPT called "games" and my domain contains the word "games". So the code does always find the slug and highlights every page and mark every menu item as anchestor. So i wrote a little fix. I just remove the domain from the menu slug. It works now perfectly.

function add_current_nav_class($classes, $item) {

    // Getting the current post details
    global $post;

    // Get post ID, if nothing found set to NULL
    $id = ( isset( $post->ID ) ? get_the_ID() : NULL );

    // Checking if post ID exist...
    if (isset( $id )){

        // Getting the post type of the current post
        $current_post_type = get_post_type_object(get_post_type($post->ID));

        // Getting the rewrite slug containing the post type's ancestors
        $ancestor_slug = $current_post_type->rewrite['slug'];

        // Split the slug into an array of ancestors and then slice off the direct parent.
        $ancestors = explode('/',$ancestor_slug);
        $parent = array_pop($ancestors);

        // Getting the URL of the menu item
        $menu_slug = strtolower(trim($item->url));

        // Remove domain from menu slug
        $menu_slug = str_replace($_SERVER['SERVER_NAME'], "", $menu_slug);

        // If the menu item URL contains the post type's parent
        if (!empty($menu_slug) && !empty($parent) && strpos($menu_slug,$parent) !== false) {
            $classes[] = 'current-menu-item';
        }
        
        // If the menu item URL contains any of the post type's ancestors
        foreach ( $ancestors as $ancestor ) {
            if (strpos($menu_slug,$ancestor) !== false) {
                $classes[] = 'current-page-ancestor';
            }
        }
    }
    // Return the corrected set of classes to be added to the menu item
    return $classes;

} add_action('nav_menu_css_class', 'add_current_nav_class', 10, 2 );

I had a problem when the rewrite slug is multiple levels, ie 'support/downloads'. I edited the code to fix this, and allow for unlimited levels, along with the ID protection that xeyefex added.

function add_current_nav_class($classes, $item) {

    // Getting the current post details
    global $post;

    // Get post ID, if nothing found set to NULL
    $id = ( isset( $post->ID ) ? get_the_ID() : NULL );

    // Checking if post ID exist...
    if (isset( $id )){

        // Getting the post type of the current post
        $current_post_type = get_post_type_object(get_post_type($post->ID));

        // Getting the rewrite slug containing the post type's ancestors
        $ancestor_slug = $current_post_type->rewrite['slug'];

        // Split the slug into an array of ancestors and then slice off the direct parent.
        $ancestors = explode('/',$ancestor_slug);
        $parent = array_pop($ancestors);

        // Getting the URL of the menu item
        $menu_slug = strtolower(trim($item->url));

        // If the menu item URL contains the post type's parent
        if (strpos($menu_slug,$parent) !== false) {
            $classes[] = 'current-menu-item';
        }

        // If the menu item URL contains any of the post type's ancestors
        foreach ( $ancestors as $ancestor ) {
            if (strpos($menu_slug,$ancestor) !== false) {
                $classes[] = 'current-page-ancestor';
            }
        }
    }
    // Return the corrected set of classes to be added to the menu item
    return $classes;

} add_action('nav_menu_css_class', 'add_current_nav_class', 10, 2 );

This solution is 99% there for me, but doesn't quite work when there are other items that may share part of the CPT slug in their slugs. For example, the slug for my CPT is /my-hr/knowledge/....

In our Nav, we have the main My HR item, but then this has a couple of sub pages in a drop down.

My HR
-- Contact HR
-- Employee Benefits

With this above function, both Contact and Employee benefits show active as well as My HR when viewing a CPT post.

Anyone know a workaround for this?

Thanks for sharing!

Further fixes I came up with:

Added a fallback to empty string if rewrite is false

$ancestor_slug = $current_post_type->rewrite ? $current_post_type->rewrite['slug'] : '';

Check for $menu_slug & $anchestor

if (!empty($menu_slug) && !empty($ancestor) && strpos($menu_slug,$ancestor) !== false) {

Result:

function add_current_nav_class($classes, $item) {

   // Getting the current post details
   global $post;

   // Get post ID, if nothing found set to NULL
   $id = ( isset( $post->ID ) ? get_the_ID() : NULL );

   // Checking if post ID exist...
   if (isset( $id )){

       // Getting the post type of the current post
       $current_post_type = get_post_type_object(get_post_type($post->ID));

       // Getting the rewrite slug containing the post type's ancestors
       $ancestor_slug = $current_post_type->rewrite ? $current_post_type->rewrite['slug'] : '';

       // Split the slug into an array of ancestors and then slice off the direct parent.
       $ancestors = explode('/',$ancestor_slug);
       $parent = array_pop($ancestors);

       // Getting the URL of the menu item
       $menu_slug = strtolower(trim($item->url));

       // Remove domain from menu slug
       $menu_slug = str_replace($_SERVER['SERVER_NAME'], "", $menu_slug);

       // If the menu item URL contains the post type's parent
       if (!empty($menu_slug) && !empty($parent) && strpos($menu_slug,$parent) !== false) {
           $classes[] = 'current-menu-item';
       }
       
       // If the menu item URL contains any of the post type's ancestors
       foreach ( $ancestors as $ancestor ) {
           if (!empty($menu_slug) && !empty($ancestor) && strpos($menu_slug,$ancestor) !== false) {
               $classes[] = 'current-page-ancestor';
           }
       }
   }
   // Return the corrected set of classes to be added to the menu item
   return $classes;

}
add_action('nav_menu_css_class', 'add_current_nav_class', 10, 2 );

@mmoollllee (and others) thanks! I updated the original gist 👍

Thank you. Exactly what I needed! :) 👍

Thank you, this is working great:) I'm surprised Wordpress doesn't do that out of the box...

Great Gist, thanks to everyone for sharing! Saved me a lot of time!

It did however not work for me with child pages that are an ancestor of a menu item.
I think it's because pages don't have a rewrite slug.

So I checked for pages and populated the ancestors array accordingly

if ( $current_post_type->name == 'page'){
	$ancestors = array();
	foreach ( get_post_ancestors($post) as $ancestor ) {
		$ancestors[] = get_post_field( 'post_name', $ancestor );
	}
}

This is the full snippet:

function add_current_nav_class($classes, $item) {

	// Getting the current post details
	global $post;

	// Get post ID, if nothing found set to NULL
	$id = ( isset( $post->ID ) ? get_the_ID() : NULL );

	// Checking if post ID exist...
	if (isset( $id )){

		// Getting the post type of the current post
		$current_post_type = get_post_type_object(get_post_type($post->ID));
		//var_dump($current_post_type->name);

		// Getting the rewrite slug containing the post type's ancestors
		$ancestor_slug = $current_post_type->rewrite ? $current_post_type->rewrite['slug'] : '';

		// Split the slug into an array of ancestors and then slice off the direct parent.
		$ancestors = explode('/',$ancestor_slug);

		// Pages have no rewrite base, so check for parent pages and populate array with all slugs of parents
		if ( $current_post_type->name == 'page'){
			$ancestors = array();
			foreach ( get_post_ancestors($post) as $ancestor ) {
				$ancestors[] = get_post_field( 'post_name', $ancestor );
			}
		}

		$parent = array_pop($ancestors);

		// Getting the URL of the menu item
		$menu_slug = strtolower(trim($item->url));

		// Remove domain from menu slug
		$menu_slug = str_replace($_SERVER['SERVER_NAME'], "", $menu_slug);

		// If the menu item URL contains the post type's parent
		if (!empty($menu_slug) && !empty($parent) && strpos($menu_slug,$parent) !== false) {
			$classes[] = 'current-menu-item';
		}

		// If the menu item URL contains any of the post type's ancestors
		foreach ( $ancestors as $ancestor ) {
			if (!empty($menu_slug) && !empty($ancestor) && strpos($menu_slug,$ancestor) !== false) {
				$classes[] = 'current-page-ancestor';
			}
		}
	}
	// Return the corrected set of classes to be added to the menu item
	return $classes;
}
add_action('nav_menu_css_class', 'add_current_nav_class', 10, 2 );