I need to do something I'm told that is relatively easy to do, but I can't find specific instructions on how to do it.

I need a way to look at incoming emails and run commands based on key words in them.

For example, incoming email:

Subject: Thing# 123
Body: delivered

when sent to dostuff@mydomain.com will run the command:

php -q ./app.php 123 setstatus="delivered"

...somebody mentioned something about cron and sendmail, blah blah. but I'm a noob, I need step-by-step help.

I'm using Apache on a remotely hosted account, I have SSH access.