Example on how to send a message using Guzzle via XboxAPI

XboxAPI Send Message.php

<?php

require 'vendor/autoload.php';

use GuzzleHttp\Pool;
use GuzzleHttp\Client;
use GuzzleHttp\Psr7\Request;

$client = new Client();

$request = new Request(
        "POST",
        "https://xboxapi.com/v2/messages",
        [
            "Content-Type" => "application/json",
            "X-Auth" => "XboxAPI API KEY",
        ],
        json_encode([
            'to' => [
                '2533274813081462',
            ],
            'message' => 'Test Message',
        ])
    );

$response = $client->send($request);

echo "Response HTTP : " . $response->getStatusCode() . PHP_EOL;