php - if else 查询，我需要添加以限制用户输入高于库存的数量

Question

我想限制用户输入高于库存的数量。产品数量来自不同的表，下面的数量是购物车中产品的数量。

执行此操作的最佳方法是什么？

这是我的add_cart.php

<?php
    include('session.php');
    if(isset($_POST['cart'])){
        $id=$_POST['id'];
        $qty=$_POST['qty'];

        $query=mysqli_query($conn,"select * from cart where productid='$id' and userid='".$_SESSION['id']."'");
        if (mysqli_num_rows($query)>0){
            echo "Product already on your cart!";
        }
        else{
            mysqli_query($conn,"insert into cart (userid, productid, qty) values ('".$_SESSION['id']."', '$id', '$qty')");
        }
    }

?>

Answer 1

这仅用于演示目的

<?php

    $query=mysqli_query($conn,"select * from cart where productid='$id' and userid='".$_SESSION['id']."'");

    if (mysqli_num_rows($query)>0){
        echo "Product already on your cart!";
    }
    else{
        //GET STOCK
        $query=mysqli_query($conn,"select * from table where stock = '$yourstock' ");
        $row=mysqli_fetch_array($query);
        //CHECK IF QTY IS HIGHER THAN STOCK OR NOT
        if($qty > $row['stock'] ){
            //SHOW ERROR THAT VALUE IS HIGHER THAN STOCK
        }else{
            mysqli_query($conn,"insert into cart (userid, productid, qty) values ('".$_SESSION['id']."', '$id', '$qty')");
        }
    }