Author: Ben Rothman

function custom_user_taxonomy() {

  $labels = array(
    'name'                       => _x( 'Departments', 'Departments Name', 'text_domain' ),
    'singular_name'              => _x( 'Department', 'Department Name', 'text_domain' ),
    'menu_name'                  => __( 'Departments', 'text_domain' ),
    'all_items'                  => __( 'All Departments', 'text_domain' ),
    'parent_item'                => __( 'Parent Department', 'text_domain' ),
    'parent_item_colon'          => __( 'Parent Department:', 'text_domain' ),
    'new_item_name'              => __( 'New Department Name', 'text_domain' ),
    'add_new_item'               => __( 'Add Department', 'text_domain' ),
    'edit_item'                  => __( 'Edit Department', 'text_domain' ),
    'update_item'                => __( 'Update Department', 'text_domain' ),
    'view_item'                  => __( 'View Department', 'text_domain' ),
    'separate_items_with_commas' => __( 'Separate department with commas', 'text_domain' ),
    'add_or_remove_items'        => __( 'Add or remove departments', 'text_domain' ),
    'choose_from_most_used'      => __( 'Choose from the most used', 'text_domain' ),
    'popular_items'              => __( 'Popular Departments', 'text_domain' ),
    'search_items'               => __( 'Search Departments', 'text_domain' ),
    'not_found'                  => __( 'Not Found', 'text_domain' ),
    'no_terms'                   => __( 'No departments', 'text_domain' ),
    'items_list'                 => __( 'Departments list', 'text_domain' ),
    'items_list_navigation'      => __( 'Departments list navigation', 'text_domain' ),
  );
  $args = array(
    'labels'                     => $labels,
    'hierarchical'               => true,
    'public'                     => true,
    'show_ui'                    => true,
    'show_admin_column'          => true,
    'show_in_nav_menus'          => true,
    'show_tagcloud'              => true,
  );
  register_taxonomy( 'departments', 'user', $args );

}
add_action( 'init', 'custom_user_taxonomy', 0 );

/**
* Admin page for the 'departments' taxonomy
*/
function cb_add_departments_taxonomy_admin_page() {

 $tax = get_taxonomy( 'departments' );

 add_users_page(
esc_attr( $tax->labels->menu_name ),
esc_attr( $tax->labels->menu_name ),
$tax->cap->manage_terms,
'edit-tags.php?taxonomy=' . $tax->name
 );

}
add_action( 'admin_menu', 'cb_add_departments_taxonomy_admin_page' );

function cb_edit_user_department_section( $user ) {
  global $pagenow;

  $tax = get_taxonomy( 'departments' );

  /* Make sure the user can assign terms of the departments taxonomy before proceeding. */
  if ( !current_user_can( $tax->cap->assign_terms ) )
    return;

  /* Get the terms of the 'departments' taxonomy. */
  $terms = get_terms( 'departments', array( 'hide_empty' => false ) ); ?>

  <h3><?php _e( 'Departments' ); ?></h3>

  <table class="form-table">

    <tr>
      <th><label for="departments"><?php _e( 'Allocated Departments' ); ?></label></th>

      <td><?php

      /* If there are any departments terms, loop through them and display checkboxes. */
      if ( !empty( $terms ) ) {
          echo cb_custom_form_field('departments', $terms, $user->ID);
      }

      /* If there are no departments terms, display a message. */
      else {
        _e( 'There are no departments available.' );
      }

      ?></td>
    </tr>

  </table>
<?php }

add_action( 'show_user_profile', 'cb_edit_user_department_section' );
add_action( 'edit_user_profile', 'cb_edit_user_department_section' );
add_action( 'user_new_form', 'cb_edit_user_department_section' );

/**
 * @param int $user_id The ID of the user to save the terms for.
 */
function cb_save_user_department_terms( $user_id ) {

  $tax = get_taxonomy( 'departments' );

  /* Make sure the current user can edit the user and assign terms before proceeding. */
  if ( !current_user_can( 'edit_user', $user_id ) && current_user_can( $tax->cap->assign_terms ) )
    return false;

  $term = $_POST['departments'];
  $terms = is_array($term) ? $term : (int) $term; // fix for checkbox and select input field

  /* Sets the terms (we're just using a single term) for the user. */
  wp_set_object_terms( $user_id, $terms, 'departments', false);

  clean_object_term_cache( $user_id, 'departments' );
}

add_action( 'personal_options_update', 'cb_save_user_department_terms' );
add_action( 'edit_user_profile_update', 'cb_save_user_department_terms' );
add_action( 'user_register', 'cb_save_user_department_terms' );