nextcloud-server/settings/ajax/togglesubadmins.php

<?php
/**
 * @author Bart Visscher <bartv@thisnet.nl>
 * @author Georg Ehrke <georg@owncloud.com>
 * @author Lukas Reschke <lukas@owncloud.com>
 * @author Morris Jobke <hey@morrisjobke.de>
 * @author Thomas Müller <thomas.mueller@tmit.eu>
 *
 * @copyright Copyright (c) 2015, ownCloud, Inc.
 * @license AGPL-3.0
 *
 * This code is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License, version 3,
 * as published by the Free Software Foundation.
 *
 * 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, version 3,
 * along with this program.  If not, see <http://www.gnu.org/licenses/>
 *
 */
OC_JSON::checkAdminUser();
OCP\JSON::callCheck();

$username = (string)$_POST['username'];
$group = (string)$_POST['group'];

// Toggle group
if(OC_SubAdmin::isSubAdminofGroup($username, $group)) {
	OC_SubAdmin::deleteSubAdmin($username, $group);
}else{
	OC_SubAdmin::createSubAdmin($username, $group);
}

OC_JSON::success();