Get a list of all of the tags used on a custom post type of a certain category

wp-tags-for-category-cpt-with-caching.php

<?php
/*
 * Assumes your custom post type is called `sma_members`, and that your category has ID `70`.
 * You'd want to chage this if you reused these.
 */

function get_cached_sma_members_tags() {
    $cached_tags = get_transient('sma_members_category_70_tags');

    if (false !== $cached_tags) {
        return $cached_tags;
    }

    $tags = fetch_sma_members_tags();
    
    set_transient('sma_members_category_70_tags', $tags, 0);

    return $tags;
}

function fetch_sma_members_tags() {
    $args = array(
        'post_type' => 'sma_members',
        'cat' => 70,
        'posts_per_page' => -1,
    );

    $query = new WP_Query($args);
    $tag_ids = array();

    if ($query->have_posts()) {
        while ($query->have_posts()) {
            $query->the_post();
            $post_tags = wp_get_post_tags(get_the_ID());
            foreach ($post_tags as $tag) {
                $tag_ids[] = $tag->term_id;
            }
        }
    }

    wp_reset_postdata();

    $unique_tag_ids = array_unique($tag_ids);

    return get_tags(array(
        'include' => $unique_tag_ids,
        'hide_empty' => false,
    ));
}



// This handles flushing the WP Transients cache whenever a post of the type is updated (better than a TTL expiration)
function refresh_sma_members_tags_cache($post_id) {
    if( get_post_type($post_id) === 'sma_members' ){
        $tags = fetch_sma_members_tags();
        set_transient( 'sma_members_category_70_tags', $tags, 0 );
    }
}
add_action('save_post_sma_members', 'refresh_sma_members_tags_cache');
add_action('deleted_post', 'refresh_sma_members_tags_cache');
add_action('edited_terms', 'refresh_sma_members_tags_cache');
add_action('delete_term', 'refresh_sma_members_tags_cache');



// Usage
$tags = get_cached_sma_members_tags();

if (!empty($tags)) {
    echo '<ul>';
    foreach ($tags as $tag) {
        echo '<li>' . $tag->name . '</li>';
    }
    echo '</ul>';
} else {
    echo 'No tags found.';
}
?>