General developer forum

That code is quite insecure. You can not just grab directly variables from POST or GET, This would be a bit better version

$users   = optional_param_array('userid'   , [], PARAM_INT);
$courses = optional_param_array('course_id', [], PARAM_INT);

/**
 * @param \enrol_plugin $enrol
 * @param stdClass $course
 * @return bool|mixed|null
 */
function get_enrol_instance(\enrol_plugin $enrol, $course) {
    global $DB;
    $instances = enrol_get_instances($course->id, true);
    $ename = $enrol->get_name();
    $enrolinstance = null;
    foreach ($instances as $instance) {
        if ($instance->enrol == $ename) {
            $enrolinstance = $instance;
            break;
        }
    }
    if ($enrolinstance === null) {
        $instanceid = $enrol->add_default_instance($course);
        if ($instanceid === null) {
            $instanceid = $enrol->add_instance($course);
        }
        if (!empty($instanceid)) {
            $enrolinstance = $DB->get_record('enrol', array('id' => $instanceid));
        }
    }

    return $enrolinstance;
}

function check_enrol(array $courseids, array $userids, $roleid, $enrolmethod = 'manual', $timestart = 0, $timeend = 0) {
    global $DB;
    $result = false;
    if (!$DB->record_exists('role', ['id' => $roleid])) {
        return false;
    }
    $enrol = enrol_get_plugin($enrolmethod);
    if ($enrol === null) {
        return $result;
    }
    list($sql, $params)  = $DB->get_in_or_equal($userids, SQL_PARAMS_NAMED);
    $params['deleted'] = false;
    $validuids = array_keys($DB->get_records_select_menu('user', 'deleted = :deleted AND id '.$sql, $params, '', 'id'));
    foreach ($courseids as $courseid) {
        $course = get_course($courseid);
        $context = context_course::instance($courseid);
        $enrollinstance = get_enrol_instance($enrol, $course);
        if ($enrollinstance !== null) {
            foreach ($validuids as $userid) {
                if (!is_enrolled($context, $userid)) {
                    $enrol->enrol_user($enrollinstance, $userid, $roleid, $timestart, $timeend);
                } else {
                    $enrol->update_user_enrol($enrollinstance, $userid, ENROL_USER_ACTIVE, $timestart, $timeend);
                }
            }
            $result = true;
        }
    }
    return $result;
}