Having worked with pcntl fork in a number of projects i do not believe there is any way to send data back to the parent process directly. You would be able to do this via the database as you have already mentioned however you may be better off using PHP's shared memory components (http://php.net/manual/en/book.shmop.php) or memcache for this purpose.
Can you elaborate on what you are doing, it may be that you can avoid this requirement.