code: protected function headers(array $server)
      {
          $headers = array();
  
          foreach ((array) $server as $key => $value) {
              $http = strpos($key, 'HTTP_') === 0;
  
              $string = strtolower(substr($key, 5));
  
              $string = str_replace('_', ' ', $string);
  
              $string = ucwords(strtolower($string));
  
              $key = str_replace(' ', '-', $string);
  
              $http && $headers[$key] = $value;
          }
  
          return $headers;
      }
docs: Converts $_SERVER parameters to message header values.
  
  @param  array $server
  @return array