PiwikTracker::getVisitorId PHP Method

    public function getVisitorId()
    {
        if (!empty($this->userId)) {
            return $this->getUserIdHashed($this->userId);
        }
        if (!empty($this->forcedVisitorId)) {
            return $this->forcedVisitorId;
        }
        if ($this->loadVisitorIdCookie()) {
            return $this->cookieVisitorId;
        }
        return $this->randomVisitorId;
    }

 /**
  * Test setting/getting the first party cookie via the PHP Tracking Client
  * @param $t
  */
 private function testFirstPartyCookies(PiwikTracker $t)
 {
     $domainHash = $this->getFirstPartyCookieDomainHash();
     $idCookieName = '_pk_id_1_' . $domainHash;
     $refCookieName = '_pk_ref_1_' . $domainHash;
     $customVarCookieName = '_pk_cvar_1_' . $domainHash;
     $viewts = '1302307497';
     $uuid = 'ca0afe7b6b692ff5';
     $_COOKIE[$idCookieName] = $uuid . '.1302307497.1.' . $viewts . '.1302307497';
     $_COOKIE[$refCookieName] = '["YEAH","RIGHT!",1302307497,"http://referrer.example.org/page/sub?query=test&test2=test3"]';
     $_COOKIE[$customVarCookieName] = '{"1":["VAR 1 set, var 2 not set","yes"],"3":["var 3 set","yes!!!!"]}';
     // test loading 'id' cookie
     self::assertContains("_viewts=" . $viewts, $t->getUrlTrackPageView());
     self::assertEquals($uuid, $t->getVisitorId());
     self::assertEquals($t->getAttributionInfo(), $_COOKIE[$refCookieName]);
     self::assertEquals(array("VAR 1 set, var 2 not set", "yes"), $t->getCustomVariable(1));
     self::assertFalse($t->getCustomVariable(2));
     self::assertEquals(array("var 3 set", "yes!!!!"), $t->getCustomVariable(3));
     self::assertFalse($t->getCustomVariable(4));
     self::assertFalse($t->getCustomVariable(5));
     self::assertFalse($t->getCustomVariable(6));
     self::assertFalse($t->getCustomVariable(-1));
     unset($_COOKIE[$idCookieName]);
     unset($_COOKIE[$refCookieName]);
     unset($_COOKIE[$customVarCookieName]);
 }