如何利用PHP开发买菜系统的商品库存预警功能？

<?php
require_once('db.php');
require_once('config.php');
?>

<?php
function getInventory($product_id) {
    $db = new DB();
    $query = "SELECT * FROM products WHERE id='$product_id'";
    $result = $db->query($query);
    if($result->num_rows > 0) {
        $row = $result->fetch_assoc();
        return $row['inventory'];
    } else {
        return 0;
    }
}
?>

<?php
function updateInventory($product_id, $inventory) {
    $db = new DB();
    $query = "UPDATE products SET inventory='$inventory' WHERE id='$product_id'";
    $db->query($query);
}
?>

<?php
function getInventoryAlert($product_id) {
    $db = new DB();
    $query = "SELECT * FROM products WHERE id='$product_id'";
    $result = $db->query($query);
    if($result->num_rows > 0) {
        $row = $result->fetch_assoc();
        return $row['inventory_alert'];
    } else {
        return 0;
    }
}

function updateInventoryAlert($product_id, $inventory_alert) {
    $db = new DB();
    $query = "UPDATE products SET inventory_alert='$inventory_alert' WHERE id='$product_id'";
    $db->query($query);
}
?>

<?php
$product_id = $_GET['product_id'];

$inventory = getInventory($product_id);
$inventory_alert = getInventoryAlert($product_id);

if($inventory <= $inventory_alert) {
    echo "商品库存不足，请及时采购！";
} else {
    echo "商品库存充足。";
}
?>