When Attempting to Filter Plugin Generated Content Using Filter post_type_link, Permalinks Are Not Modified

Question

(My issue is not that described in 'post_type_link' filter not working. In that instance Rizwan was attempting to use post_type_link to filter posts, which are instead handled by post_link.)

In my case I'm trying to use post_type_link on a custom post type created by the Liquid Messages plugin. The post type is 'gc-sermons' and my code is as follows:

function lqd_message_link( $url, $post ) {
  if ( get_post_type( $post ) == 'gc-sermons' && isset( $wp_query->query_vars['messages-app-view'] ) ) {
    return add_query_arg( $_GET, $url );
    }
  return $url;
  }
add_filter( 'post_type_link', 'lqd_message_link', 10, 2 );

If you go to a sermon series page like the below:

https://liquidchurch.com/sermon-series/divine-direction/messages-app-view/

And then mouse over one of the messages you see the link URL is:

https://liquidchurch.com/message/step-by-step

Whereas I'm looking for it to be:

https://liquidchurch.com/message/step-by-step/messages-app-view/

Any ideas?

Note: The CPT (gc-sermons) is set during its registration with ep_mask as EP_ALL.

Answer 1

Milo provided the answer in the comments above. I'm recording for posterity sake. :)

I was using add_query_arg and should instead have used return $url, so the code above should have looked like:

function lqd_message_link( $url, $post ) {
   global $wp_query;
   if ( get_post_type( $post ) == 'gc-sermons' && isset( $wp_query->query_vars['messages-app-view'] ) ) {
   return $url . 'messages-app-view/';
}
   return $url;
}
add_filter( 'post_type_link', 'lqd_message_link', 10, 2);