Making a list of post tags in string

I am trying to push post tags to the search index of the WPFTS plugin in order to search my posts by tag names. To achieve this, I have to generate a string with all tags. Here is what I tried:

add_filter('wpfts_index_post', function($index, $post)
{
    global $wpdb;
    $data = get_the_tags($post->ID);
    $index['post_tag'] = implode(' ', $data);
    return $index;
}, 3, 2);

The problem is I see this new search cluster ‘post_tag’ in the list, but all data there is empty. What did I do wrong?

You’re using get_the_tags() incorrectly. It does not return an array of tag names. It returns an array of tag objects. Each tag object does contain the tag’s name (along with other data), but you can’t use implode() on that result.

I used your original code just adding a step to put the tag object’s name into an array. Then your code picks up with the implode from there.

add_filter('wpfts_index_post', function($index, $post)
{
    global $wpdb;
    $data = get_the_tags($post->ID);

    $tag_array = array();
    foreach( $data as $tag_object ) {
        $tag_array[] = $tag_object->name;
    }

    $index['post_tag'] = implode(' ', $tag_array);
    return $index;
}, 3, 2);

Always check the documentation for a function to make sure you are getting the expected result (in this case an object instead of an array). See get_the_tags().