Nuevo usuario alta

<?php
include 'connection.php';

$mysqli->stmt_init();

$Username_Alta = $_POST['Username'];
$Email = $_POST['Email'];
$PIN = $_POST['PIN'];
$pass = $_POST['Password'];
$Password = password_hash($pass, PASSWORD_BCRYPT);

$stmt = $mysqli->prepare("SELECT * FROM Usuarios WHERE Email = ?");
$stmt->bind_param('s', $Email);

if($result = $stmt->execute()){

    while ($row = $result->fetch_assoc()) {
        $PIN_DB = $row['PIN'];
    }

    if ($PIN == $PIN_DB) {
        $stmt = $mysqli->prepare('UPDATE Usuarios SET Username=?, Password=? WHERE PIN=?;');
        $stmt->bind_param('sss', $Username_Alta, $Password, $PIN);

        if ($stmt->execute()) {
            $stmt = $mysqli->prepare('UPDATE Usuarios SET PIN="" WHERE Username=?;');
            $stmt->bind_param('s', $Username_Alta);
            $stmt->execute();
            Header("Location: https://".$_SERVER["SERVER_NAME"]."/cliente?Confirmar_Alta&Alta_Ok");
        }else{
            Header("Location: https://".$_SERVER["SERVER_NAME"]."/cliente?Confirmar_Alta&Alta_Error");
        }
    }
} else {
    Header("Location: https://".$_SERVER["SERVER_NAME"]."/cliente?Confirmar_Alta&DB_Error");
}

mysqli_close($mysqli);
?>