如何解決使用 MySQLi 準備語句時 PHP「指令不同步」錯誤？

<?php

$mysqli = new mysqli('localhost', 'user', 'password', 'database');

// Prepare and execute first statement
$stmt1 = $mysqli->prepare("SELECT * FROM users WHERE username = ?");
$stmt1->bind_param('s', $username);
$stmt1->execute();
$stmt1->store_result();

// Free the result set and advance to the next result
$stmt1->free_result();
$mysqli->next_result();

// Prepare and execute second statement
$stmt2 = $mysqli->prepare("SELECT * FROM orders WHERE user_id = ?");
$stmt2->bind_param('i', $userId);
$stmt2->execute();
$stmt2->store_result();

// Bind results and fetch data
$stmt2->bind_result($orderId, $productId, $quantity);
while ($stmt2->fetch()) {
    // Do something with the fetched data
}

$stmt2->free_result();
$mysqli->close();

?>