PHP substract 'error' if a-b<0

Question

Im worink with the 'shop-app' and I want to get error response when a-b<0 , because if user have for example 10$ and an item cost 20$, he cant buy it, but in my app it look that: 10-20=-10 Here is php script:

<?php
if($_SERVER['REQUEST_METHOD']=='POST'){

include 'DatabaseConfig.php';

$con = mysqli_connect($HostName,$HostUser,$HostPass,$DatabaseName);

$id = $_POST['UserID'];
$S_cash = $_POST['Cash'];
$S_substract = $_POST['Substract'];



$Sql_Query = "UPDATE user SET cash= '$S_cash' - '$S_substract' WHERE id = $id";

if(mysqli_query($con,$Sql_Query))
{
echo 'Success!';
}
else
if('$S_cash' - '$S_substract' < 0)
///this is what ive tried
{
 echo 'error';
}
}
mysqli_close($con);
?>

The problem is you are working with strings not numbers. Convert your strings into numbers and do the math. Also, try to read on how to properly use user input without SQL injection. — Ibu, May 11 '18 at 23:04
if you have a real shop, please please please read up on prepared statement, escaping and/or sanitizing input, sql injections (as linked in the related links) — Jakumi, May 11 '18 at 23:55

electricjelly · Answer 1 · 2018-05-11T23:21:14.440

1

your putting '$S_cash' - '$S_substract' in single quotes and because of that php is interpreting it as a literal $S_cash - $S_substract

instead of the numbers they stand for.

try using double quotes as in

"$S_cash" - "$S_substract"

or no quotes at all as in

$S_cash - $S_substract

also consider using prepared statements

edited May 11 '18 at 23:21

answered May 11 '18 at 23:03

electricjelly

366
1
2
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MadLordDev · Answer 2 · 2018-05-13T23:25:36.660

1

I don't know why you are using

if('$S_cash' - '$S_substract' < 0)

this. normally it would be

if($S_cash - $S_substract < 0)

plus, looking at your code. You are checking whether your response is correct, by outputing success here

if(mysqli_query($con,$Sql_Query))
{
echo 'Success!';
}

and if it is not, then you are using your if statement

else
if('$S_cash' - '$S_substract' < 0)
///this is what ive tried
{
 echo 'error';
}

I don't know if you mean it or it's error. Maybe you want to get rid of else statement

edited May 13 '18 at 23:25

answered May 11 '18 at 23:03

MadLordDev

260
2
12

I would like to receive an error that will prevent the user update if a-b<0 – Master May 11 '18 at 23:14
I understand that but why are you using else after getting true from query? Plus, you should be checking if user does not have enough money before updating your value in database. – MadLordDev May 11 '18 at 23:27
Ur right, i didnt try it yet – Master May 11 '18 at 23:52

PHP substract 'error' if a-b<0

2 Answers2