It is very easy to programmatically update your twitter status using PHP. Here is the ready to use code that can be used to make auto-tweet; may be you can write the custom code that fetches random blog post from your blog and tweet it on your twitter. It is currently making use of twitter basic authentication. It is good to use OAUTH. However, this basic authentication is still valid.

$username = ‘twitterusername’;
$password = ‘twitterpassword’;
$format = ‘xml’; //alternative: json
$message = $message . urlencode($url);
$result = shell_exec(‘curl http://twitter.com/statuses/update.’.$format.’ -u ‘.$username.’:’.$password.’ -d status=”‘.str_replace(‘”‘,’\”‘,$message).’”‘);
echo $result;