Php flutter curl komutu

Merhaba,

Php de flutter projeesine curl ile bildirim göndermeye çalışıyorum. Sürekli InvalidRegistration hatası alıyorum. Yardımcı olabilecek var mı?

{"multicast_id":4226262067472702987,"success":0,"f ailure":1,"canonical_ids":0,"results":[{"error":"InvalidRegistration"}]}

$api_key="AAAAydjeNKA:APA91bH_HOMSA4eWYiEDFS_rSOiN05-DKu9Sal3cw0I2tSc2kE1NrwBx6tPVAsqd_2aTo_57iJ6GwTUwF8abcgh7mKX5loBvpB5GZF9jv4g6ZbqRtHbDXPzW6OcmbO";
            
            $url="https://fcm.googleapis.com/fcm/send";

            $data=array(
                'title'=>'Greetings',
                'body'=>'Hi, From PHP Script'
            );
            
            $fields=json_encode(array(
                'to'=>'topics/all',
                'notification'=>$data,
                'priority'=>'high'
            ));

            // Generated by curl-to-PHP: http://incarnate.github.io/curl-to-php/
            $ch = curl_init();

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

            curl_setopt($ch, CURLOPT_URL, $url);
            curl_setopt($ch, CURLOPT_POST, 1);
            curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
            curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
            curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);  
           curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
            curl_setopt($ch, CURLOPT_POSTFIELDS, ($fields));

            curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($fields));
               $result = curl_exec($ch);          
               if ($result === FALSE) {
                   die('Curl failed: ' . curl_error($ch));
               }
               curl_close($ch);



            $result = curl_exec($ch);
            if (curl_errno($ch)) {
                echo 'Error:' . curl_error($ch);
            }
            var_dump($result);
            curl_close($ch);