Send emails asynchronously with a queue in PHP

Approach

Don't send email inside the web request. Record the intent in the database (an outbox table) in the same transaction as the business change, and let a worker deliver it with retries and backoff. Use a library like Symfony Mailer for transport. Frameworks provide queues (Symfony Messenger, Laravel Queues) that implement this for you.

Solution

<?php
declare(strict_types=1);
// Inside the request: same transaction as the order
$pdo->beginTransaction();
$orderId = $orders->create($cart);
$pdo->prepare('INSERT INTO outbox (type, payload, available_at) VALUES (?, ?, NOW())')
    ->execute(['order_receipt', json_encode(['order_id' => $orderId])]);
$pdo->commit();

// worker.php (run under systemd/supervisor)
use Symfony\Component\Mailer\Mailer;
use Symfony\Component\Mailer\Transport;
use Symfony\Component\Mime\Email;

$mailer = new Mailer(Transport::fromDsn(getenv('MAILER_DSN')));
while (true) {
    $pdo->beginTransaction();
    $job = $pdo->query("SELECT * FROM outbox WHERE sent_at IS NULL AND available_at <= NOW()
                        ORDER BY id LIMIT 1 FOR UPDATE SKIP LOCKED")->fetch(PDO::FETCH_ASSOC);
    if (!$job) { $pdo->commit(); sleep(1); continue; }
    try {
        $data = json_decode($job['payload'], true, flags: JSON_THROW_ON_ERROR);
        $mailer->send(buildReceiptEmail($data['order_id']));
        $pdo->prepare('UPDATE outbox SET sent_at = NOW() WHERE id = ?')->execute([$job['id']]);
    } catch (Throwable $e) {
        $delay = min(3600, 2 ** (int) $job['attempts'] * 30);
        $pdo->prepare('UPDATE outbox SET attempts = attempts + 1, last_error = ?, available_at = NOW() + make_interval(secs => ?) WHERE id = ?')
            ->execute([substr($e->getMessage(), 0, 500), $delay, $job['id']]);
    }
    $pdo->commit();
}

Pitfalls

• Sending emails before the transaction commits (email for a rolled-back order).
• No max attempts / dead-letter handling.
• Workers without a supervisor, so they silently stop.

Variations

• Symfony Messenger with the Doctrine or Redis transport and retry_strategy.
• Laravel: Mail::to($user)->queue(new OrderReceipt($order)) with php artisan queue:work.

Further reading: Symfony Messenger