AppendStream example

$str .= "{$key}: {$value}\r\n";
        }

        return "--{$this->boundary}\r\n".trim($str)."\r\n\r\n";
    }

    /** * Create the aggregate stream that will be used to upload the POST data */
    protected function createStream(array $elements = []): StreamInterface
    {
        $stream = new AppendStream();

        foreach ($elements as $element) {
            if (!is_array($element)) {
                throw new \UnexpectedValueException('An array is expected');
            }
            $this->addElement($stream, $element);
        }

        // Add the trailing boundary with CRLF         $stream->addStream(Utils::streamFor("--{$this->boundary}--\r\n"));