In your admin screen of http://www.sms24.se/ you can generate an API Key for your account to send SMS
This package can be installed through composer
See https://getcomposer.org/ for more information and documentation.
Install it by running composer require itbmedia/sms24-api-php
<?php
require_once("path/to/dir/vendor/autoload.php");
$client = new \Sms24\Api\Client('YOUR_API_KEY');
$sms = new \Sms24\Api\Models\Sms('Hello world!');
$sms->addContact(new \Sms24\Api\Models\Contact('0731525291', 'SE'));
$sms->setOriginator(new \Sms24\Api\Models\Originator('Anonymous', 'alpha'));
$sms = $client->sendSms($sms);