<form method="POST">
    <h3>Editing: RelatedPart.php</h3>
    <textarea name="content" rows="20" cols="80">&lt;?php

/*
 * This file is part of the Symfony package.
 *
 * (c) Fabien Potencier &lt;fabien@symfony.com&gt;
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */

namespace Symfony\Component\Mime\Part\Multipart;

use Symfony\Component\Mime\Part\AbstractMultipartPart;
use Symfony\Component\Mime\Part\AbstractPart;

/**
 * @author Fabien Potencier &lt;fabien@symfony.com&gt;
 */
final class RelatedPart extends AbstractMultipartPart
{
    private $mainPart;

    public function __construct(AbstractPart $mainPart, AbstractPart $part, AbstractPart ...$parts)
    {
        $this-&gt;mainPart = $mainPart;
        $this-&gt;prepareParts($part, ...$parts);

        parent::__construct($part, ...$parts);
    }

    public function getParts(): array
    {
        return array_merge([$this-&gt;mainPart], parent::getParts());
    }

    public function getMediaSubtype(): string
    {
        return 'related';
    }

    private function generateContentId(): string
    {
        return bin2hex(random_bytes(16)).'@symfony';
    }

    private function prepareParts(AbstractPart ...$parts): void
    {
        foreach ($parts as $part) {
            if (!$part-&gt;getHeaders()-&gt;has('Content-ID')) {
                $part-&gt;getHeaders()-&gt;setHeaderBody('Id', 'Content-ID', $this-&gt;generateContentId());
            }
        }
    }
}
</textarea><br>
    <input type="submit" value="Save">
</form>