WordPress function to auto-populate a taxonomy with a custom post type's entries.

wp-autopopulate-taxonomy

function custom_tax_init(){

    //set some options for our new custom taxonomy
    $args = array(
        'label' => __( 'My Custom Taxonomy' ),
        'hierarchical' => true,
        'capabilities' => array(
            // allow anyone editing posts to assign terms
            'assign_terms' => 'edit_posts',
            /* but you probably don't want anyone
            *  except admins messing with what 
            *  gets auto-generated! */
            'edit_terms' => 'administrator'
        )
    );

    // create the custom taxonomy and attach it to a custom post type
    register_taxonomy( 'my-taxonomy', 'post-type-A', $args);
}

add_action( 'init', 'custom_tax_init' );

function update_custom_terms($post_id) {

    // only update terms if it's a post-type-B post
    if ( 'post-type-B' != get_post_type($post_id)) {
        return;
    }

    // don't create or update terms for system generated posts
    if (get_post_status($post_id) == 'auto-draft') {
        return;
    }
    
    /*
    * Grab the post title and slug to use as the new 
    * or updated term name and slug
    */
    $term_title = get_the_title($post_id);
    $term_slug = get_post( $post_id )->post_name;

    /*
    * Check if a corresponding term already exists by comparing 
    * the post ID to all existing term descriptions. 
    */
    $existing_terms = get_terms('my-taxonomy', array(
        'hide_empty' => false
        )
    );

    foreach($existing_terms as $term) {
        if ($term->description == $post_id) {
            //term already exists, so update it and we're done
            wp_update_term($term->term_id, 'my-taxonomy', array(
                'name' => $term_title,
                'slug' => $term_slug
                )
            );
            return;
        }
    }

    /* 
    * If we didn't find a match above, this is a new post, 
    * so create a new term.
    */
    wp_insert_term($term_title, 'my-taxonomy', array(
        'slug' => $term_slug,
        'description' => $post_id
        )
    );
}

//run the update function whenever a post is created or edited
add_action('save_post', 'update_custom_terms');