public static function validateCaFile($filename, LoggerInterface $logger = null)
    {
        static $warned = false;

        if (isset(self::$caFileValidity[$filename])) {
            return self::$caFileValidity[$filename];
        }

        $contents = file_get_contents($filename);

        // assume the CA is valid if php is vulnerable to
        // https://www.sektioneins.de/advisories/advisory-012013-php-openssl_x509_parse-memory-corruption-vulnerability.html
        if (!static::isOpensslParseSafe()) {
            if (!$warned && $logger) {
                $logger->warning(sprintf(
                    'Your version of PHP, %s, is affected by CVE-2013-6420 and cannot safely perform certificate validation, we strongly suggest you upgrade.',
                    PHP_VERSION
                ));
                $warned = true;
            }

            $isValid = !empty($contents);
        } else {
            $isValid = (bool) openssl_x509_parse($contents);
        }

        if ($logger) {
            $logger->debug('Checked CA file '.realpath($filename).': '.($isValid ? 'valid' : 'invalid'));
        }

        return self::$caFileValidity[$filename] = $isValid;
    }