Firebase notification PHP script

Question

I'm testing firebase notification since my old project was integrated to GCM, but now I'm also using firebase database.

I'm trying to execute a php script in order to send notification to one of my device. I'm able to obtain the TokenID and also I'm able to send notifications to this token from the Firebase Console, but when I use the PHP script, I have the following error:

Invalid (legacy) Server-key delivered or Sender is not authorized to perform request.

Here is my script:

$message = $_POST['message'];
$title = $_POST['title'];
$path_to_fcm = 'https://fcm.googleapis.com/fcm/send';
$server_key = "AAAXXXXXX";
$key =  "c43xxxxxxxxx";


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

$fields = array('to'=>$key,
            'notification'=>array('title'=>$title,'body'=>$message));

$payload = json_encode($fields);

$curl_session = curl_init();
curl_setopt($curl_session, CURLOPT_URL, $path_to_fcm);
curl_setopt($curl_session, CURLOPT_HTTPHEADER, $headers);
curl_setopt($curl_session, CURLOPT_POSTFIELDS, $payload);
curl_setopt($curl_session, CURLOPT_CUSTOMREQUEST, "POST"); 

$result = curl_exec($curl_session);
curl_close($curl_session);

echo $result;

I'm getting the api key from:

Firebase console > gear icon > proyect configuration > Cloud messaging

Answer 1

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

);

you should make the server_key a variable like $server_key