There's a few tutorials and classes out there but they no longer appear to work since Twitter changed to oAuth.
How do I send a tweet from PHP, in the most basic way possible?
For example, the 'old' way using the MyTwitter class was as follows:
require_once('MyTwitter.class.php');
$twitter = new MyTwitter('username', 'password');
$message = "API Test.";
$result = $twitter->updateStatus($message);
print_r($result);