PHP 使用Fetch API POST JSON資料，Transaction交易為例

Javascript 部分

fetch('./purchase.php', { 
  method: 'POST',
  headers: {
    'Content-Type': 'application/json'
  },
  body: JSON.stringify({
    'product_id': 1,
    'order_amount': 2,
  })
}).then(response => response.text()).then((body) => {
  console.log(body);
});

PHP 部分

$content = trim(file_get_contents("php://input"));
$decoded = json_decode($content, true);
$product_id = $decoded['product_id'];
$order_amount = $decoded['order_amount'];
$conn->autocommit(FALSE);
$conn->begin_transaction();
$result = $conn->query('SELECT amount FROM products WHERE id = $product_id for update'); 
$row = $result1->fetch_assoc();
// 確定庫存大於購買數目
if ($row['amount'] >= $order_amount) {
  $conn->query("UPDATE products SET amount = amount - $order_amount WHERE id = $product_id");
  echo '購買成功'; // echo 回傳值會出現在 JS 中的 then(response)
} else {
  echo '購買失敗';
}
$conn->commit();
$conn->close();

參考資料：https://codepen.io/dericksozo/post/fetch-api-json-php