If you want to reinvent the wheel then good luck to you but the sort of service you're looking for is available here:
http://posterous.com/
Otherwise you'll need to look at the API for each of the site's you're talking about. I've sucessfully pushed content to Tumblr and edited it, though NOT from XAMPP on localhost, only by doing it directly from the web so there must be a security issue in there.
The main issue is that you'd need to store your login credentials on your site somewhere (bad if you got hacked) or you'd need to use OAuth stuff, such as this:
http://www.jaisenmathai.com/blog/2009/03/31/how-to-quickly-integrate-with-twitters-oauth-api-using-php/
In all honesty I'd say to stick to Posterous.