Calling Firebase Cloud Messaging from server does not work - php

Question

I just developed a firebase web app server. I am sending notifications and everything seems okay but in my android app no notifications are received. If I send a notification from firebase console it comes to my android app. However I want to send it from my web app.
Here is my php code for web app:
public function sendMessage($data,$target=null){
//FCM api URL
$url = 'https://fcm.googleapis.com/fcm/send';
//api_key available in Firebase Console -> Project Settings -> CLOUD MESSAGING -> Server key
$server_key = 'S6GzaCUhIbS6GzaCAIzaSyJtkQLzl3FCUhIbUhIbS6Gza8A';
//$fields = array();
//$fields['data'] = $data;
if(is_array($target)){
$fields['registration_ids'] = $target;
}
$fields = array(
'notification' => array('title' => 'Working Good', 'body' => 'That is all we want'),
'data' => array('message' => "melaapa")
);
//header with content_type api key
$headers = array(
'Content-Type:application/json',
'Authorization:key='.$server_key
);
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($fields));
$result = curl_exec($ch);
if ($result === FALSE) {
die('FCM Send Error: ' . curl_error($ch));
}
curl_close($ch);
var_dump($result);
return $result;
}
The output of
var_dump($result)
is
string(3) "to "
What should the reponse be for a working firebase server or how can I understand that my firebase server is working fine?