error bagian row

ini error bagian row kira2 apa ya ? saya memakai php 7

 <?php
//host, user, password, database

$db = new mysqli("localhost","root","","belajar");

if($db->connect_errno){
  echo 'error gan !'.$db->connect_error;
}
$hasil ="SELECT * FROM murid";
$data= $db->query($hasil);
if ($data->num_rows >0){
 while($row = $data->fetch_assoc()){
 echo $row[id];
 echo $row[nama];
 echo $row[senjata]."<br>";
 }
}else{
echo 'data kosong';
}
$db->close();
?>