WP_REST_Users_Controller::create_item PHP Method

    public function create_item($request)
    {
        if (!empty($request['id'])) {
            return new WP_Error('rest_user_exists', __('Cannot create existing user.'), array('status' => 400));
        }
        $schema = $this->get_item_schema();
        if (!empty($request['roles']) && !empty($schema['properties']['roles'])) {
            $check_permission = $this->check_role_update($request['id'], $request['roles']);
            if (is_wp_error($check_permission)) {
                return $check_permission;
            }
        }
        $user = $this->prepare_item_for_database($request);
        if (is_multisite()) {
            $ret = wpmu_validate_user_signup($user->user_login, $user->user_email);
            if (is_wp_error($ret['errors']) && !empty($ret['errors']->errors)) {
                $error = new WP_Error('rest_invalid_param', __('Invalid user parameter(s).'), array('status' => 400));
                foreach ($ret['errors']->errors as $code => $messages) {
                    foreach ($messages as $message) {
                        $error->add($code, $message);
                    }
                    if ($error_data = $error->get_error_data($code)) {
                        $error->add_data($error_data, $code);
                    }
                }
                return $error;
            }
        }
        if (is_multisite()) {
            $user_id = wpmu_create_user($user->user_login, $user->user_pass, $user->user_email);
            if (!$user_id) {
                return new WP_Error('rest_user_create', __('Error creating new user.'), array('status' => 500));
            }
            $user->ID = $user_id;
            $user_id = wp_update_user(wp_slash((array) $user));
            if (is_wp_error($user_id)) {
                return $user_id;
            }
            add_user_to_blog(get_site()->id, $user_id, '');
        } else {
            $user_id = wp_insert_user(wp_slash((array) $user));
            if (is_wp_error($user_id)) {
                return $user_id;
            }
        }
        $user = get_user_by('id', $user_id);
        /**
         * Fires immediately after a user is created or updated via the REST API.
         *
         * @since 4.7.0
         *
         * @param WP_User         $user     Inserted or updated user object.
         * @param WP_REST_Request $request  Request object.
         * @param bool            $creating True when creating a user, false when updating.
         */
        do_action('rest_insert_user', $user, $request, true);
        if (!empty($request['roles']) && !empty($schema['properties']['roles'])) {
            array_map(array($user, 'add_role'), $request['roles']);
        }
        if (!empty($schema['properties']['meta']) && isset($request['meta'])) {
            $meta_update = $this->meta->update_value($request['meta'], $user_id);
            if (is_wp_error($meta_update)) {
                return $meta_update;
            }
        }
        $user = get_user_by('id', $user_id);
        $fields_update = $this->update_additional_fields_for_object($user, $request);
        if (is_wp_error($fields_update)) {
            return $fields_update;
        }
        $request->set_param('context', 'edit');
        $response = $this->prepare_item_for_response($user, $request);
        $response = rest_ensure_response($response);
        $response->set_status(201);
        $response->header('Location', rest_url(sprintf('%s/%s/%d', $this->namespace, $this->rest_base, $user_id)));
        return $response;
    }