By Daniel15 (dan.cx) This is a very simple Twitter autoresponder bot. It requires PECL OAuth extension to be installed (run "pecl install oauth", or if on Windows, grab php-oauth.dll. If using cPanel you can install it via WHM). The authentication is designed for command-line usage, it won't work too well via a web browser. You'll have to sign up for an application on Twitter's site to get the consumer key and secret.
Could be modified to be more advanced (match regular expressions to answer questions, etc.)
Questions? See my blog post - http://dan.cx/blog/2011/06/twitter-autoreply-bot-dbznappa
Modified 2013-06-13 - Twitter API 1.0 discontinued, modified to use Twitter API 1.1
@BenPalmer have you checked that it is successfully created the file 'since_id'? And the contents?
Anyway, the reason I was here was to thank Daniel for posting this - it just gave me a few leads to get something very similar up using Twitter-Async. I also dropped a conditional ... if (!$tweet->in_reply_to_status_id) ... into the innermost foreach loop so that it didn't auto-reply to @replies, only to brand new tweets. Nice and simple.