code: public static function getPurgeSignHeader(Config $serviceConfig, $urlString)
      {
          $gmtDate = gmdate('D, d M Y H:i:s \G\M\T');
          $sign = md5("$urlString&{$serviceConfig->serviceName}&$gmtDate&{$serviceConfig->operatorPassword}");
          return array(
              'Authorization' => "UpYun {$serviceConfig->serviceName}:{$serviceConfig->operatorName}:$sign",
              'Date' => $gmtDate,
              'User-agent' => 'Php-Sdk/' . $serviceConfig->getVersion() . ' (purge api)'
          );
      }
docs: 获取请求缓存刷新接口需要的签名头
  
  @param Config $serviceConfig
  @param $urlString
  
  @return array