I would advise you to crypt your data with a very long encryption key which only your host and your server knows.
You can use this encryption/decryption class (StackOverflow) from John Conde.
For the sake of simplicity I would then simply serialize your
$data array, crypt it and send it to your server.
$urlData = Encryption::encrypt(serialize($data)); curl_setopt($ch,CURLOPT_POSTFIELDS,array('data' => $urlData));
On side of your FAX server just decrypt the data and unserialize it again
$data = unserialize(Encryption::decrypt($dataParameterFromUrl));
Server-side you know have the exact same data you had on your client and can do whatever you want with it.
Furthermore you could use something like challenge response to verify your host.