Piwik\Plugins\VisitsSummary\API::getUniqueVisitors PHP Method

    public function getUniqueVisitors($idSite, $period, $date, $segment = false)
    {
        $metric = 'nb_uniq_visitors';
        $this->checkUniqueIsEnabledOrFail($period, $metric);
        return $this->getNumeric($idSite, $period, $date, $segment, $metric);
    }