How to get valid firebase token in android?

I am trying to send push notification but when code gets executed it shows invalidresgistration error. Below is the code

LoginActivity

 String getToken = FirebaseInstanceIDService.onTokenRefreshs();
String getUsername = username.getText().toString();
String URL = DataHolder.TokenUrl(getUsername,getToken);

OnTokenRefresh code

public class FirebaseInstanceIDService extends FirebaseInstanceIdService {
    public static String onTokenRefreshs(){
        String token = FirebaseInstanceId.getInstance().getToken();
        return  token;
    }
}

pushnotification code

$url = 'https://fcm.googleapis.com/fcm/send';

          $message = array("message" => "hell man");

     $xyz=array("firebasetokengoeshere I haven't included for now");
     $fields = array(
     'to' => json_encode($xyz),
     'data' => $message
     );

     $headers = array(
     'Authorization:key = Appkey goes here',
     'Content-type: application/json'
     );

     $ch = curl_init();
     curl_setopt($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){

     }
     curl_close($ch);

     return $result;



     $message_status = send_notification($tokens, $message);
     echo $message;

{"multicast_id":6744250798651355841,"success":0,"failure":1,"canonical_ids":0,"results":[{"error":"InvalidRegistration"}]}

I am getting above result. How to fix this error.