Firebase如何发送主题通知

我使用下面的脚本向特定用户发送通知:

 "abc",
    'title'     => "Hello from Api",
    'vibrate'   => 1,
    'sound'     => 1,
);

$fields = array
(
    'registration_ids'  => $registrationIds,
    'notification'          => $msg
);

$headers = array
(
    'Authorization: key=' . API_ACCESS_KEY,
    'Content-Type: application/json'
);

$ch = curl_init();
curl_setopt( $ch,CURLOPT_URL, 'https://fcm.googleapis.com/fcm/send' );
curl_setopt( $ch,CURLOPT_POST, true );
curl_setopt( $ch,CURLOPT_HTTPHEADER, $headers );
curl_setopt( $ch,CURLOPT_RETURNTRANSFER, true );
curl_setopt( $ch,CURLOPT_SSL_VERIFYPEER, false );
curl_setopt( $ch,CURLOPT_POSTFIELDS, json_encode( $fields ) );
$result = curl_exec($ch );
curl_close( $ch );
echo $result;
?>

脚本工作正常,但我如何向安装我的应用程序的所有用户发送通知.我在我的应用程序(警报)中创建了一个主题,我可以通过firebase控制台向所有用户发送通知.任何人都可以指导我更新上面的主题脚本.

我通过替换修复

$fields = array
(
    'registration_ids'  => $registrationIds,
    'notification'          => $msg
);

至

$fields = array
(
    'to'  => '/topics/alerts',
    'notification'          => $msg
);