code: private function addHeaderValue(HttpHeaders $headers, string $name, $value, bool $append): void
      {
          $decodedValue = trim((string)(isset(self::$headersToUrlDecode[$name]) ? urldecode($value) : $value));
  
          if (isset(self::$specialCaseHeaders[$name])) {
              $headers->add($name, $decodedValue, $append);
          } elseif (strpos($name, 'HTTP_') === 0) {
              // Drop the "HTTP_"
              $normalizedName = substr($name, 5);
              $headers->add($normalizedName, $decodedValue, $append);
          }
      }
docs: Adds a header value
  
  @param HttpHeaders $headers The headers to add to
  @param string $name The name of the header
  @param mixed $value The header value to add
  @param bool $append Whether or not to append the value