The following code works perfectly...
$message = $client->account->sms_messages->create($twilio_number, $to, $body, array("StatusCallback" => "http://etc...));
...for text messages within the 160 character limit. The SMS is sent, and my server is contacted at the callback URL when the status changes.
However, this method doesn't facilitate concatenated messages or MMS. For those, the Twilio documentation gives an example of sendMessage. This code works...
$message = $client->account->messages->sendMessage($from, $to, $body, $mediaURL);
...but the fourth call parameter, formerly used for the StatusCallBack URL, is replaced by the Media URL.
The Twilio documentation page has an "Optional Parameters" section, in which StatusCallback is listed and explained, but there's no example of how to include it when using the sendMessage method shown above and in their sample code.
Is it possible to specify a callback using the sendMessage method, and if so, how is it done?