openpgp-php/lib/OpenPgP/SignaturePacket/Subpacket.php

<?php

namespace Leenooks\OpenPGP\SignaturePacket;

use Leenooks\OpenPGP\Exceptions\PacketTagException;
use Leenooks\OpenPGP\Packet;

class Subpacket extends Packet
{
	protected $tag = NULL;

	protected function header_and_body(): array
	{
		return ['header'=>$this->size().chr($this->tag),'body'=>$this->body()];
	}

	/* Defaults for unsupported packets */
	function read()
	{
		$this->data = $this->input;
	}

	public function setTag(int $tag): void
	{
		if (get_class($this) !== Subpacket::class)
			throw new PacketTagException('Attempting to set a tag for invalid class: ',get_class($this));

		$this->tag = $tag;
	}

	protected function size(): string
	{
		// Use 5-octet lengths + 1 for tag as first packet body octet
		return chr(255).pack('N',strlen($this->body())+1);
	}
}