optional parameter in permalink structure

Question

Im currently facing an issue with the relationship between pretty permalink and my themes templates.

I have a post type called 'performance', and with pretty permalinks enabled the URLs for the template at single-performance.php looks like this:

www.my-website.com/performance/name-of-performance

In the sidebar of that template, I am listing 5 other performances. This all works fine so far. Standard WordPress stuff.

On another page I am listing all the performances that plays during school hours. This is done with a WP_Query filtering with a meta_query. Still fine.

But this is where I get stuck: When you click one of the school performances it links to the template single-performance.php, and this template has 5 other performances listed in the sidebar. But these are not filtered by school hours. They just show the normal 5 performances.

Is it possible to have an optional parameter in the url so that if it looks like this:

www.my-website.com/performance/name-of-performance

... it lists the normal 5 performances in the sidebar. But if it looks like this:

www.my-website.com/performance/schools/name-of-performance 

... it lists 5 performances filtered by the school hours.

In a world without permalink I would simple have this url

www.my-website.com/page_id=12&schools=1

Then in the template i would go

if( isset($_GET['schools'] ) && $_GET['schools'] == 1 ) {
  // Show performances filtered by school hours in sidebar
} else {
  // Show 5 performances without filtering them
}

Please let me know if this is possible, and if it is, what approach to take.

I have been looking at add_permastruct() add_query_vars filter and add_rewrite_rules() but it all confuses me, as most examples are not solving what I am trying to solve.

Answer 1

Update

You can make use of the add_rewrite_endpoint:

add_rewrite_endpoint( 'schools', EP_PERMALINK | EP_PAGES );

You can then check in the sidebar for the schools variable:

global $wp_query;

if ( isset( $wp_query->query_vars['schools'] ) ) {
    // ...
}

What is worth mentioning is that your URL will look like www.example.com/performance/name-of-performance/schools/ instead of having schools immediately after the custom post type's name.