I'm comfortable with theme hooks, regularly add features, but struggling with a plugin hook, to filter content. It's at the limit of my knowledge, and I'm trying to understand the process, to no avail.

I'm using GigPress to manage events. It allows you to add a show (date based) and relate that show to a post (standard WP post).

I've created a custom post type — productions — to relate shows to, instead of just the generic blog posts.

The plugin has a hook — gigpress_related_post_types — to allow you to swap posts for any CPT of your choice.

If I edit the plugin directly, it works, swapping "posts" for "productions". If I add what I hope to be a correct function, it doesn't break anything, but instead ALL posts and posts types are available to choose. Suggesting I'm overriding default behavior, but incorrectly calling my function?

The sample plugin code:

$related_posts_sql = "SELECT p.ID, p.post_title FROM " . $wpdb->prefix . "posts p WHERE (p.post_status = 'publish' OR p.post_status = 'future')";
 * Provides an opportunity to specify other post types as related posts
 * @param array $related_post_types
 * @since 2.3.19

$related_post_types = apply_filters('gigpress_related_post_types', ['post']);
if (!empty($related_post_types)) {
  $related_posts_sql .= "AND p.post_type IN( '" . implode("','", $related_post_types) . "' )";

$related_posts_sql .= " ORDER BY p.post_date DESC LIMIT 500";

$entries = $wpdb->get_results($related_posts_sql, ARRAY_A);
if ($entries != FALSE) {
  foreach ($entries as $entry) { ?>
      value="<?php echo $entry['ID']; ?>"<?php if (isset($show_related) && $entry['ID'] == $show_related) {
      echo(' selected="selected"');
      $found_related = TRUE;
    } ?>><?php echo gigpress_db_out($entry['post_title']); ?></option>
  <?php }

Full plugin new show file here: https://codeshare.io/50MKJg

An example of one of the many functions I've been suggested is:

add_filter( 'gigpress_related_post_types', array( 'productions' ) ); if ( ! empty( $related_post_types ) ) { $related_posts_sql .= "AND p.post_type IN( '" . implode( "','", $related_post_types ) . "' )"; }

Ideally, I would also like to offer two CPTs in place of "posts" — both "productions" and "projects".

  • 1
    Undefined variables in your code: $wpdb and $show_related. Consider getting a proper IDE like PHPStorm. It has WordPress support and coding standards built-in.
    – leymannx
    Apr 17, 2019 at 10:01
  • Thanks @leymannx — that's the plugin itself, and only a sample, relating to teh post types... The plugin author suggested only the hook "gigpress_related_post_types" was needed to target it in functions, but can't offer support or details. Apr 17, 2019 at 10:38
  • What’s your code that you’re using to try and change the value. Apr 17, 2019 at 10:56
  • Thanks @JacobPeattie... I've tried a million variations... [well, ten+?) Things along the lines of, have been suggested to me: add_filter( 'gigpress_related_post_types', array( 'productions' ) ); if ( ! empty( $related_post_types ) ) { $related_posts_sql .= "AND p.post_type IN( '" . implode( "','", $related_post_types ) . "' )"; } Apr 17, 2019 at 11:07

1 Answer 1


You're on the right path with your add_filter, but this is the right way to implement filters.

add_filter( 'gigpress_related_post_types', 'my_related_post_types' ); 
function my_related_post_types( $post_types ) {
    return array( 'productions' );

The hook add_filter must call a function that returns something to pass to the apply_filters in the code you referenced.

You should add in whatever logic you need to return the correct post types and handle the default as well. For example:

add_filter( 'gigpress_related_post_types', 'my_related_post_types' ); 
function my_related_post_types( $post_types ) {
    // Assuming you set an option to control which post types should be used.
    $custom_gig_post_types = get_option( 'gig_post_types', false );
    if ( $custom_gig_post_types ) {
        return $custom_gig_post_types;
    return $post_types;
  • You're a legend, thank you so much! Not only does it work, but I understand why, I think! Can I buy you a beer?! Apr 17, 2019 at 15:20
  • Haha. I appreciate the offer. Happy to help. Apr 17, 2019 at 15:26

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