Sending SMS messages via php sdk

I'm trying to use the php sdk to send an sms message to a group of folks, but I can't run a successful test. Here's my code. What am I doing wrong?

$recipient = "tel:11235554321";
$message = "Hi, this is a test message.";
$sender = "011235551234";

try{
//createMessage requires an array of recipients
$message_id = $ribbit->Messages()->createMessage(array($recipient), $message, $sender);
$result = "You have sent message (id " . $message_id .")";
}
catch (RibbitException $e){
$result = "An error occured - status " . $e->getStatus() . " - " . $e->getMessage();
}

/** END RIBBIT STUFF **/