public static function get_cohort_members($cohortids) {
        global $DB;
        $params = self::validate_parameters(self::get_cohort_members_parameters(), array('cohortids' => $cohortids));

        $members = array();

        foreach ($params['cohortids'] as $cohortid) {
            // Validate params.
            $cohort = $DB->get_record('cohort', array('id' => $cohortid), '*', MUST_EXIST);
            // Now security checks.
            $context = context::instance_by_id($cohort->contextid, MUST_EXIST);
            if ($context->contextlevel != CONTEXT_COURSECAT and $context->contextlevel != CONTEXT_SYSTEM) {
                throw new invalid_parameter_exception('Invalid context');
            }
            self::validate_context($context);
            if (!has_any_capability(array('moodle/cohort:manage', 'moodle/cohort:view'), $context)) {
                throw new required_capability_exception($context, 'moodle/cohort:view', 'nopermissions', '');
            }

            $cohortmembers = $DB->get_records_sql("SELECT u.id FROM {user} u, {cohort_members} cm
                WHERE u.id = cm.userid AND cm.cohortid = ?
                ORDER BY lastname ASC, firstname ASC", array($cohort->id));
            $members[] = array('cohortid' => $cohortid, 'userids' => array_keys($cohortmembers));
        }
        return $members;
    }