code: public function add($name, $values, bool $append = false): void
      {
          $normalizedName = self::normalizeHeaderName($name);
  
          if (!$append || !$this->containsKey($normalizedName)) {
              parent::add($normalizedName, (array)$values);
          } else {
              $currentValues = [];
              $this->tryGet($normalizedName, $currentValues);
              parent::add($normalizedName, array_merge($currentValues, (array)$values));
          }
      }
docs: Headers are allowed to have multiple values, so we must add support for that
  
  @inheritdoc
  @param string|array $values The value or values
  @param bool $append Whether or not to append the value to to the other header values