Firebase clould FCM API using

1. Go to Google API Console Menu -> IAM and Admin -> Service accounts
2. Find your service and create key. Don’t forget to choose your project at top:
3. Create key for your service, choose json type.
4. You will need your downloaded conf file in PHP google api client in function setAuthConfig.
5. Install Google API Client.

PHP code:

$client = new Google_Client();
$client->setAuthConfig('client_secrets.json');
$client->setAccessType("offline");        // offline access
$client->setIncludeGrantedScopes(true);   // incremental auth
$client->addScope(array(
   "https://www.googleapis.com/auth/cloud-platform",
   "https://www.googleapis.com/auth/firebase.messaging")); // our scopes to use Firebase notifications api
$client->setRedirectUri('http://' . $_SERVER['HTTP_HOST'] . '/index.php'); // redirect if need
print_r($client->fetchAccessTokenWithAssertion()); // print access token or get to variable

1. Next we need to send post request to google api fcm. Docs here in tab recent.
2. Make curl request:

$url = 'https://fcm.googleapis.com/v1/projects/test-notifications-a0093/messages:send'; //test-notifications-a0093 it’s your app id in firebase project settings
$fields = [
    'message' => [
        'notification' => [
            'title' => 'test title 123123',
            'body' => 'test body asdasd',
        ],
        'data' => [
            'tab' => "2",
        ],
        'android' => [
            'collapse_key' => 'test',
            'priority' => 'high',
            'notification' => [
                'sound' => "default",
                'tag' => "new_messages",
            ],
        ],
        'topic' => 'all',
    ],
];
$fields = json_encode ( $fields );

$headers = array (
    'Authorization: Bearer ya29.c.ElzLBrmFbcZh7OZdlMvsHIg2ZhqZMjgD9NQ_..',// Baerer {token that you got from google clinet}
    'Content-Type: application/json'
);

$ch = curl_init ();
curl_setopt ( $ch, CURLOPT_URL, $url );
curl_setopt ( $ch, CURLOPT_POST, true );
curl_setopt ( $ch, CURLOPT_HTTPHEADER, $headers );
curl_setopt ( $ch, CURLOPT_RETURNTRANSFER, true );
curl_setopt ( $ch, CURLOPT_POSTFIELDS, $fields );

$result = curl_exec ( $ch );
echo $result;
curl_close ( $ch );