You can use a service that is connected to the GSM network. You use a simple API to send SMS messages or attach MMS images. The rates are fair.
I use TextLocal in UK and you can send messages to the world. There are allot of other service providers available too.
Simple PHP to send a text message
<?php
// Textlocal account details
$username = urlencode('[email protected]');
// Get this when logged in at https://control.txtlocal.co.uk/docs/
$hash = urlencode('Your API hash');
// Message details
$numbers = urlencode(447123456789,447987654321);
$sender = urlencode('Textlocal');
$message = urlencode('This is your message');
// Prepare data for POST request
$data = 'username=' . $username . '&hash=' . $hash
. '&numbers=' . $numbers . "&sender=" . $sender . "&message=" . $message;
// Send the GET request with cURL
$ch = curl_init('https://api.txtlocal.com/send/?' . $data);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$response = curl_exec($ch);
curl_close($ch);
// Process your response here
echo $response;
?>