stmt oop pertama

<?php

if (isset($_POST['register'])) {

$username = filter_input(INPUT_POST,'username',FILTER_SANITIZE_STRING);
$password = filter_input(INPUT_POST,'password',FILTER_SANITIZE_STRING);

if (empty($username) || empty($password) || empty($_POST['email'])) {
  echo "tolong dimasukan email username dan password";
  exit();
}

$query = "SELECT * FROM users WHERE username=:username";

$query_params = array(':username' => $username);

try {
$stmt= $db->prepare($query);
$result = $stmt->execute($query_params);

} catch (Exception $error) {
  die("failed to run query");
}
$row = $stmt->fetch();

if ($row) {
    echo "username sudah terpakai";
    exit();
}

if (!filter_var($_POST['email'],FILTER_VALIDATE_EMAIL)) {
  echo "bukan format email";
  exit();
}
$query = "SELECT * FROM users WHERE email=:email";
$query_params = array(':email' => $_POST['email']);

try {
$stmt= $db->prepare($query);
$result = $stmt->execute($query_params);

} catch (Exception $error) {
  die('failed to run query');
}
$row = $stmt->fetch();
if ($row) {
  echo "email sudah terpakai";
  exit();
}


$salt =dechex(mt_rand(0,2147483647)).dechex(mt_rand(0,2147483647));
$password = hash('sha256',$password.$salt);
for ($round=0; $round < 65536 ; $round++) {
  $pasword = hash('sha256',$password.$salt);
}


$query = "INSERT INTO users(username,email,password,salt) VALUES (:username,:email,:password,:salt)";
$query_params = array(
          ":username" => $username,
          ":email" => $_POST['email'],
          ":password" => $password,
          ":salt" => $salt,
);
try{
$stmt = $db->prepare($query);
$result = $stmt->execute($query_params);
}catch(PDOException $error){
  die('failed to run query');
}
header("Location: login.php");




}
 ?>