FCM 주제단위로 전송하기

<?php
define("GOOGLE_SERVER_KEY", "FCM에서 발급받은키";

function send_fcm($message, $id) {
    $url = 'https://fcm.googleapis.com/fcm/send';

    $headers = array (
        'Authorization: key='.GOOGLE_SERVER_KEY,
        'Content-Type: application/json'
    );
    $fields = array (
        'data' => array ("message" => $message),
        'notification' => array ("body" => $message)
    );
    if(is_array($id)) {
        $fields['registration_ids'] = $id;
    } else {
        $fields['to'] = $id;
    }
    $fields['priority'] = "high";
    $fields = json_encode ($fields);
    $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 ( $ch );
    if ($result === FALSE) {
        die('FCM Send Error: ' . curl_error($ch));
    }
    curl_close ( $ch );

    return $result;
}
$message_status= send_fcm("test", "/topics/구독한주제");

echo $message_status;
?>

php에서 주제단위로 전송할때 

"/topics/안드로이드에서 구독시킨 명을 넣어주면된다"

ps)안드로이드에서 주제 구독시키는법 FirebaseMessaging.getInstance().subscribeToTopic("구독시킬명");