Comparing from textbox to database - php

Question

Please help me. I'm having trouble in comparing
How do I compare the quantity that was inserted from the stocks from the database
<?php
session_start();
include_once("config.php");
//empty cart by distroying current session
if(isset($_GET["emptycart"]) && $_GET["emptycart"]==1)
{
$return_url = base64_decode($_GET["return_url"]); //return url
session_destroy();
header('Location:'.$return_url);
}
//add item in shopping cart
if(isset($_POST["type"]) && $_POST["type"]=='add')
{
$product_code = filter_var($_POST["product_code"], FILTER_SANITIZE_STRING); //product code
$product_qty = filter_var($_POST["product_qty"], FILTER_SANITIZE_NUMBER_INT); //product code
$return_url = base64_decode($_POST["return_url"]); //return url
$sel = mysql_query("SELECT * FROM products");
$product_stock = mysql_fetch_assoc($sel);
//limit quantity for single product
if($product_qty > $products['stock'])
{
die('<div align="center">Not enought quantity<br />Back To Products.</div>');
}

Answer 1

Your error is in this line:
if($product_qty > $products['stock'])
What is $products['stock']? Above you defined the MySQL result like that:
$product_stock = mysql_fetch_assoc($sel);
So, did you mean?
if($product_qty > $product_stock['stock'])
Also, your code is vulnerable to redirection-attacks: https://www.owasp.org/index.php/Top_10_2013-A10-Unvalidated_Redirects_and_Forwards
You need to make sure that you somehow validate the parameter return_url, by allowing only a valid list of URLs. Better would be:
$valid_urls = array(1 => 'http://link1.com', 2 => '...');
And then:
$return_url_param = (int)$_GET["return_url"];
$return_url = isset($valid_urls[$return_url_param] ? $valid_urls[$return_url_param] : 'http://defaulturl.com');

Answer 2

You are comparing $product_qty from wrong variable . Try change $products['stock'] to $product_stock['stock'].
Corrected block of code:-
<?php
//add item in shopping cart
if (isset($_POST["type"]) && $_POST["type"] == 'add') {
$product_code = filter_var($_POST["product_code"], FILTER_SANITIZE_STRING); //product code
$product_qty = filter_var($_POST["product_qty"], FILTER_SANITIZE_NUMBER_INT); //product code
$return_url = base64_decode($_POST["return_url"]); //return url
$sel = mysql_query("SELECT * FROM products");
$product_stock = mysql_fetch_assoc($sel);
//limit quantity for single product
if ($product_qty > $product_stock['stock']) {
die('<div align="center">Not enought quantity<br />Back To Products.</div>');
}
}