function sendGCM($message, $registration_ids) {
    //FCM URL
    $url = "https://fcm.googleapis.com/fcm/send";
//prepare data
$fields = array (
    'registration_ids' => array ($registration_ids),
    'data' => array ("message" => $message)
);
$fields = json_encode ( $fields );

//header data
$headers = array ('Authorization: key=<YOUR_API_KEY>', 'Content-Type: application/json');

//initiate curl request
$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 );

// execute curl request
$result = curl_exec ( $ch );

//close curl request
curl_close ( $ch );

//return output
return $result;

"multicast_id": 5359746182596118281,
"success": 0,
"failure": 1,
"canonical_ids": 0,
"results": [
{
"error": "InvalidRegistration"
}
            ]
}