Zend\Diactoros\MessageTrait::withAddedHeader PHP Method

    public function withAddedHeader($header, $value)
    {
        if (is_string($value)) {
            $value = [$value];
        }
        if (!is_array($value) || !$this->arrayContainsOnlyStrings($value)) {
            throw new InvalidArgumentException('Invalid header value; must be a string or array of strings');
        }
        HeaderSecurity::assertValidName($header);
        self::assertValidHeaderValue($value);
        if (!$this->hasHeader($header)) {
            return $this->withHeader($header, $value);
        }
        $normalized = strtolower($header);
        $header = $this->headerNames[$normalized];
        $new = clone $this;
        $new->headers[$header] = array_merge($this->headers[$header], $value);
        return $new;
    }