如何使用PHP实现商城的价格保护功能

<?php
$mysqli = new mysqli('localhost', 'username', 'passWord', 'your_database');
$productId = $_GET['product_id'];
$currentTime = date('Y-m-d H:i:s');
$sql = "SELECT * FROM price_protection WHERE product_id='$productId' AND end_time>'$currentTime'";
$result = $mysqli -> query($sql);

if ($result -> num_rows == 0) { //如果没有价格保护记录，则返回商品原有价格
    $sql = "SELECT price FROM products WHERE id='$productId'";
    $result = $mysqli -> query($sql);
    $row = $result -> fetch_assoc();
    echo $row['price'];
} else { //如果存在价格保护记录，则返回价格保护的价格
    $row = $result -> fetch_assoc();
    if ($row['old_price'] == null) {
        $row['old_price'] = $row['new_price'];
        $sql = "UPDATE price_protection SET old_price=".$row['old_price']." WHERE id=".$row['id'];
        $mysqli -> query($sql);
    }

    echo $row['old_price'];
    if ($row['new_price'] < $row['old_price']) {
        $sql = "UPDATE price_protection SET old_price=".$row['new_price'].", new_price=".$row['new_price']." WHERE id=".$row['id'];
        $mysqli -> query($sql);
    } else {
        $sql = "UPDATE price_protection SET start_time='$currentTime', end_time=DATE_ADD('$currentTime', INTERVAL 7 DAY) WHERE id=".$row['id'];
        $mysqli -> query($sql);
    }
}
?>

$(document).ready(function() {
    $.ajax({
        url: 'get_price.php',
        dataType: 'JSON',
        success: function(data) {
            if (data.success) {
               $('#price').html('<span class="old-price">¥ '+data.data.old_price+'</span>¥ '+data.data.new_price+'<i class="icon-price-protection"></i>');
            } else {
               $('#price').html('<span>¥ '+data.data.price+'</span><i class="icon-price-protection"></i>');
            }
        }
    });
});