How to affect a PHP program / mysql database from an email response

I am trying to work out how to do something. It must be possible - I've seen something similar done in multiple systems (Twitter, facebook etc.)

The basic thing I want to do is - someone leaves a comment on my website. The comment goes to my email, as well as to my MySql database. I then reply directly on my email. The reply is saved to the database, and auto-approves the user's comment.

I can't work out how to speak to my MySql / PHP system directly from an email. Also - I can't work out how to set up the inboxes?

I am guessing that I'd need to set up mailserver rules so that emails sent to xxxx-response@mydomain.com are actually redirected to a php script, where xxxx is an identifier of the original comment?

Thanks for any light that you can shed...