MailQueue PHP class

<?php

class MailQueue
{
    /* PDO object for database operations */
    private $pdo;


    public function __construct(PDO &$pdo)
    {
        /* Set the PDO object */
        $this->pdo = $pdo;
    }


    /* Public function to add a new message to the queue. Returns the message ID */
    public function add_message(array $message): int
    {
        /* Check if the message is valid, otherwise throws an exception. */
        if ($this->validate_message($message))
        {
            /* Insert the base message to the queue */
            $queue_id = $this->insert_message($message);

            /* Insert the CC(N) addresses */
            $this->insert_cc($queue_id, $message['cc']);

            /* Insert the file attachments */
            $this->insert_attachments($queue_id, $message['attachments']);
        }
        else
        {
            /* Invalid message */
            throw new Exception('Invalid message.');
        }

        return $queue_id;
    }


    private function insert_message(array $message): int
    {
        /* New message ID */
        $queue_id = NULL;

        /* Insert query */
        $query = 'INSERT INTO mailqueue.queue (queue_app_id, queue_from, queue_recipient, queue_subject, queue_text_body, queue_html_body, queue_priority, queue_added, queue_schedule) VALUES (:app_id, :from, :recipient, :subject, :text_body, :html_body, :priority, NOW(), :schedule)';

        /* Query values */
        $params = array(

        ':app_id' => $message[app_id],
        ':from' => $message['from'],
        ':recipient' => $message['recipient'],
        ':subject' => $message['subject'],
        ':text_body' => $message['text_body'],
        ':html_body' => $message['html_body'],
        ':priority' => $message['priority'],
        ':schedule' => $message['schedule']);

        /* Check for PDO Exceptions (query errors) */
        try
        {
            /* Execute the query */
            $res = $this->pdo->prepare($query, array(PDO::ATTR_CURSOR => PDO::CURSOR_FWDONLY));
            $res->execute($params);

            /* Retrieve the new message id */
            $queue_id = $this->pdo->lastInsertId();
        }
        catch (PDOException $e)
        {
            /* Throw a standard exception */
            throw new Exception('Database error: ' . $e->getMessage());
        }

        return $queue_id;
    }


    private function validate_message(array $message): bool
    {
        /* TODO */
        return TRUE;
    }


    private function insert_cc(int $queue_id, array $message)
    {
        /* TODO */
        return TRUE;
    }


    private function insert_attachments(int $queue_id, array $message)
    {
        /* TODO */
        return TRUE;
    }
}