private function validateCertificate($i, $c)
 {
     try {
         $c = new CertParser($c);
         $expiresAt = $c->getNotValidAfter();
         if (time() > $expiresAt) {
             $this->logWarning(sprintf("certificate '%s' expired at %s [%s]", $c->getName(), date("r", $expiresAt), $i));
         } elseif (time() + self::EXPIRY_WARNING_TIME > $expiresAt) {
             $this->logWarning(sprintf("certificate '%s' expires at %s [%s]", $c->getName(), date("r", $expiresAt), $i));
         }
     } catch (CertParserException $e) {
         $this->logWarning(sprintf("%s [%s]", $e->getMessage(), $i));
     }
 }