Twitter continues to becoming more and more popular these days, especially for businesses. As a business, you want to keep your followers interested, and in-tune with all your media outlets. One great way is to use Twitter's direct messages. If you follow CATS on twitter, you'll get a direct message from us right away letting you know how you can keep in touch and get all of our updates.This kind of response is helpful to build a relationship with your followers. We searched google, and there were slim pickings as far as auto-responding software, so we decided to develop this inhouse. I'll be showing you how to build this into your email.

To implement this setup, you'll need the following things:

• An email account on a Linux/UNIX system
• A twitter account registered for that user
• php-cli (or another scripting language if you want to re-implement it)
• curl

When I was asked to develop the auto-responder, my first thought was "I'm getting this email from twitter that says 'George is now following you on twitter'. I bet I could use regular expressions to solve my case, but who knows when they'll change the format of the email or something...". After some investigation, I looked at the headers of the email, and I was delightfully surprised to see:

...
X-Twitterrecipientscreenname: catsone
X-Twittersenderscreenname: George_C
X-Twitteremailtype: is_following
X-Twitterrecipientid: 16535056
X-Twittersenderid: 16600001
Errors-To: Twitter <twitter-follow-twitter=catsone.com@postmaster.twitter.com>
X-Twittercreatedat: Tue Mar 09 17:04:37 +0000 2010
Bounces-To: Twitter <twitter-follow-twitter=catsone.com@postmaster.twitter.com>
X-Twitterrecipientname: CATS App Track Sys
X-Twittersendername: George C
...

The main reason you need a unix email account is so that you can create a .forward file. This file tells your mail server to redirect your email messages to another email address, or in our case, a PHP script. The contents of the file is very simple. Assuming the user's name is twitter and our processing script is located at '/home/twitter/twitterReply.php', All we need in that file is: "| /usr/bin/php /home/twitter/twitterReply.php"
You can read more about the ".forward" file at http://www-acs.ucsd.edu/students/email/dotforward.shtml

The actual PHP script will essentially read the email headers to determine if the email is a "Following" email and the user ID of your new follower. If we can determine both, then we're going to use the Twitter API to send a message to that user. I'll leave all the code explanation to the comments since the whole script is just about 50 lines.
Note: Feel free to include HTML tags in the message such as <a href="www.example.com">My Site</a> because it's running through urlencode().

<?php
 
// Config Options
$username = 'andy';
$password = 'secret';
$message  = 'This is the message your followers will get.';
 
// Read the email message from standard input
$fp = fopen('php://stdin', 'r');
 
// These default to false
$isFollowing = false;
$userID = false;
 
// Read lines until we get past the header
while($line = fgets($fp))
{
    // We just need to match a static header signifying that they are now following us
    if (trim($line) == 'X-Twitteremailtype: is_following')
    {
        $isFollowing = true;
    }
 
    // If we haven't found the userID yet, then use a regular expression to pull out the userID
    // Sure we could have used strpos() and substr(), but I <3 regular expressions.
    if (empty($userID) && preg_match('/X-Twittersenderid: (d+)/', $line, $m))
    {
        $userID = $m[1];
    }
 
    // Once we're finished reading the headers, then we're done.
    if ($line == "\n")
    {
        break;
    }
}
 
// Close the file handle properly
fclose($fp);
 
// Make sure this email is an "Is Following" email and we found a valid userID
if ($isFollowing && $userID)
{
    // urlencode the message because it's going to be a POST variable. The userID is just a number, so it's fine.
    $message = urlencode($message);
    // Use the "which" command to find the proper path for curl.
    $curl = trim(`which curl`);
    // Finally, call curl using our username and password, the userID, and the message to the Twitter API.
    // I added a redirection to /tmp/twitter.debug.txt so we can check what the last api call returned.
    exec("$curl -u $username:$password -d 'user=$userID&text=$message' \
        http://twitter.com/direct_messages/new.xml > /tmp/twitter.debug.txt");
}
?>