gnu-social-activitypub-plugin/actions/inbox/Follow.php

<?php
/**
 * GNU social - a federating social network
 *
 * ActivityPubPlugin implementation for GNU Social
 *
 * LICENCE: This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 * @category  Plugin
 * @package   GNUsocial
 * @author    Diogo Cordeiro <diogo@fc.up.pt>
 * @author    Daniel Supernault <danielsupernault@gmail.com>
 * @copyright 2018 Free Software Foundation http://fsf.org
 * @license   http://www.fsf.org/licensing/licenses/agpl-3.0.html GNU Affero General Public License version 3.0
 * @link      https://www.gnu.org/software/social/
 */
if (!defined ('GNUSOCIAL')) {
        exit (1);
}

// Validate Object
if (!is_string ($data->object)) {
        ActivityPubReturn::error ("Invalid Object object, URL expected.");
}

// Get valid Object profile
try {
        $object_profile = new Activitypub_explorer;
        $object_profile = $object_profile->lookup ($data->object)[0];
} catch(Exception $e) {
        ActivityPubReturn::error ("Invalid Object Actor URL.", 404);
}

try {
        if (!Subscription::exists ($actor_profile, $object_profile)) {
                Subscription::start ($actor_profile, $object_profile);
                $res = array ("@context" => "https://www.w3.org/ns/activitystreams",
                          "type"   => "Follow",
                          "actor"  => $data->actor,
                          "object" => $data->object);
                ActivityPubReturn::answer ($res);
        } else {
                ActivityPubReturn::error ("Already following.", 409);
        }
} catch (Exception $e) {
        ActivityPubReturn::error ("Invalid Object Actor URL.", 404);
}