Simple SMTP server in PHP - Saves attachments - was used to get stuff from a network scanner.

server.php

<?php

include_once('vendor/autoload.php');

use ZBateson\MailMimeParser\Header\AddressHeader;
use ZBateson\MailMimeParser\MailMimeParser;
use ZBateson\MailMimeParser\Message\IMimePart;

const SMTP_RESP_220 = '220 localhost ESMTP PHP8.1 Ready';
const SMTP_RESP_221 = '221 localhost ESMTP PHP8.1 Service closing transmission channel';
const SMTP_RESP_250 = '250 OK';
const SMTP_RESP_354 = '354 Start mail input; end with <CRLF>.<CRLF>';
const SMTP_RESP_500 = '500 Syntax error, command unrecognized';
const SMTP_RESP_501 = '501 Syntax error in parameters or arguments';

/**
 * Log to screen.
 *
 * @param string $message
 *
 * @return void
 */
function logIt(string $message): void
{
    $dateTime = (new DateTime())->format(\DateTimeInterface::ATOM);
    echo "[$dateTime] $message\n";
}

// Start listening on port 25
$socket = stream_socket_server('tcp://0.0.0.0:25', $errCode, $errStr);
if (!$socket) {
    throw new RuntimeException($errStr, $errCode);
}

echo "Listening on port 25...\n";
$parser = new MailMimeParser();

while (1) {
    // Accept incoming connections
    $buffer = '';
    $client = @stream_socket_accept($socket, 5);

    if ($client) {
        logIt("ACCEPTED: " . stream_socket_get_name($client, true));

        // Send the greeting message.
        fwrite($client, SMTP_RESP_220 . "\r\n");

        while ($data = fread($client, 1024)) {
            $buffer .= $data;
            $pos = strpos($buffer, "\r\n");

            if ($pos !== false) {
                $line = substr($buffer, 0, $pos);
                $buffer = substr($buffer, $pos + 2);

                // Parse SMTP command
                $command = strtoupper(substr($line, 0, 4));
                $args = trim(substr($line, 4));

                // Handle SMTP commands
                switch ($command) {
                    case 'HELO':
                    case 'EHLO':
                        logit('GOT: HELO/EHLO');
                        fwrite($client, SMTP_RESP_250 . "\r\n");
                        break;

                    case 'MAIL':
                        logit('GOT: MAIL');
                        if (preg_match('/^FROM:\s*(<[^>]+>)/i', $args, $matches)) {
                            fwrite($client, SMTP_RESP_250 . "\r\n");
                        } else {
                            fwrite($client, SMTP_RESP_501 . "\r\n");
                        }
                        break;

                    case 'RCPT':
                        logIt('GOT: RCPT');
                        if (preg_match('/^TO:\s*(<[^>]+>)/i', $args, $matches)) {
                            fwrite($client, SMTP_RESP_250 . "\r\n");
                        } else {
                            fwrite($client, SMTP_RESP_501 . "\r\n");
                        }
                        break;

                    case 'DATA':
                        logIt('GOT: DATA');
                        fwrite($client, SMTP_RESP_354 . "\r\n");

                        // Create a temporary memory stream to hold the email data.
                        $tempStream = fopen('php://temp', 'r+');

                        // Copy the email data from the client socket stream to the temporary memory stream.
                        stream_copy_to_stream($client, $tempStream);

                        // Reset the temporary memory stream pointer to the beginning.
                        rewind($tempStream);

                        // Parse the email data incrementally using the MailMimeParser.
                        $message = $parser->parse($tempStream, true);

                        echo "{$message->getHeader('Message-ID')}\n";
                        echo "{$message->getHeader('Subject')}\n";
                        echo "{$message->getHeader('From')}\n";

                        /** @var AddressHeader | null $to */
                        $to = $message->getHeader('To');
                        foreach ($to->getAddresses() as $address) {
                            echo "TO: {$address->getName()} {$address->getEmail()}\n";
                        }

                        // Save all attachments.
                        /** @var IMimePart[] $attachments */
                        $attachments = $message->getAllAttachmentParts();
                        $attachmentCount = count($attachments);
                        $loopIndex = 1;
                        foreach ($attachments as $index => $part) {
                            $filename = $part->getHeaderParameter(
                                'Content-Type',
                                'name',
                                $part->getHeaderParameter(
                                    'Content-Disposition',
                                    'filename',
                                    '__unknown_file_name_' . $index,
                                ),
                            );

                            $out = fopen(__DIR__ . '/data/' . $filename, 'w+');
                            $str = $part->getBinaryContentResourceHandle();
                            stream_copy_to_stream($str, $out);
                            fclose($str);
                            fclose($out);

                            echo "ATTACHMENT $loopIndex: $filename\n";
                            ++$loopIndex;
                        }

                        $response = "250 Message received\r\n";
                        fwrite($client, $response);
                        logIt('DATA FINISHED');
                        break;

                    case 'QUIT':
                        logIt('GOT: QUIT');
                        fwrite($client,  SMTP_RESP_221 . "\r\n");
                        break;
                    default:
                        logIt("GOT: UNKNOWN [$line]");
                        $response = SMTP_RESP_500 . "\r\n";
                        fwrite($client, $response);
                }
            }
        }

        fclose($client);

        logIt('Disconnected');
    }
}

This was just a quick script. I'm working on making a full on library (that works) for this. This has spawned a couple other projects for other libraries for concepts that I haven't see before.