Depending on your platform (Windows, *Nix), you would just want to use some scheduling system to invoke the PHP job either directly or via a local web call.
cron would be your *Nix tool of choice
A scheduled task in windows would work there.
Outside of that, you'd just need to use PHP's file classes, specifically copy, and whatever code you'd need for your cloud provider.
EDIT: examples for RackSpace api
These guys have done basically exactly what you need.
http://clloud.com/tutorials-and-tips/uploading-files-to-rackspace-cloud-using-php-aka-mosso/
I would build an upload script from that knowledge that does what you want. Then just use cron or scheduled task to invoke it on your schedule