Sync two posts if both use carbon fields association

carbon_fields_sync_posts.php

<?php
/**
 * Sync the association field between a post and the associated
 * post_tpes. To be used during carbon_fields_post_meta_container_saved
 *
 * @param string $post_id   The post id of the post that is being saved
 * @param string $key       The carbon fields key of the association field
 * @param array $target     carbon fields 'key' and 'post_type' of the target
 * @return void
 */
function carbon_sync_posts( $post_id, $key, $target ) {
    $source_post_type = get_post_type( $post_id );
    //// Get field values
    $source_values = carbon_get_post_meta( $post_id, $key);
    //// The source value field
    $source_value = 'post:' . $source_post_type . ':' . $post_id;
    //// Get all targets
    $target_posts = get_posts([
        'post_type'         =>  $target['post_type'],
        'numberposts'       => -1,
        'posts_per_page'    => -1
    ]);

    foreach( $target_posts as $post ) {
        $target_value = 'post:' . $target['post_type'] . ':' . $post->ID;
        $target_values = carbon_get_post_meta( $post->ID, $target['key'] );
        $entry_exists_in_target = count( array_filter( $target_values, function( $entry ) use ($source_value){
            return $entry['value'] == $source_value;
        })) > 0;
        $entry_should_be_in_target = count( array_filter( $source_values, function( $entry ) use ($target_value){
            return $entry['value']  == $target_value;
        })) > 0;

        //// Add if not in target
        if ( ! $entry_exists_in_target && $entry_should_be_in_target ) {
            carbon_set_post_meta( $post->ID, $target['key'], [ 'value' => $source_value ] );
        }

        //// Remove if in target but removed from source
        if ( $entry_exists_in_target && ! $entry_should_be_in_target ) {
            $new_entries = array_map( function( $entry ) use ( $source_value ) {
                if ( $entry['value'] !== $source_value ) {
                    return [ 'value' => $entry['value'] ];
                }
            }, $target_values);
            carbon_set_post_meta( $post->ID, $target['key'], $new_entries);
        }
    }
}