Send push notification using Firebase

push.php

<?php

use GuzzleHttp;

$access_token = 'FIREBASE CLOUD MESSAGING SERVER KEY';

$reg_id = 'DEVICE REGISTRATION ID';

$message = [
    'notification' => [
        'title' => 'Test Message',
        'body' => "This is a test!"
    ],
    'to' => $reg_id
];

$client = new GuzzleHttp\Client([
    'headers' => [
        'Content-Type' => 'application/json',
        'Authorization' => 'key='.$access_token,
    ]
]);

$response = $client->post('https://fcm.googleapis.com/fcm/send',
    ['body' => json_encode($message)]
);

echo $response->getBody();

Thanks