I am not sure and application that sites on your offline computer can do that but there is cron job on PHP servers, its like an alarm clock, you can set when it should launch PHP server, this will be done whether you are online or offline at the time you set it to initiate.

mike you can do that using Easyphp. Check the lateste version of it , you have apache , php , phpmyadmin ...anything you might need is in this 'software' .