Apply an amount towards a total

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I have a item and users apply amounts towards the total price. It works ok except if the item is $100 and previously users have applied $40 in total then they want to apply another amount of $80 I want only $60 to go for that item (insert) and the $20 remaining to stay in the balance. My attempt incorrectly shows in the insert (sorry math is not my strong suit).

(('-80' - (-80 - 100.00))

PHP

$applied = mysqli_real_escape_string($connect, $_POST['applied']);
$uid = mysqli_real_escape_string($connect, $_POST['uid']);

$updateStr = "select (ce.total - SUM(ad.g_price)) AS gross from       mytotal as ce JOIN transactions as ad ON ce.id=ad.pid where ce.uid ='$applied'";


$uQuery = mysqli_query($connect,$updateStr);
$totalArray = mysqli_fetch_array($uQuery, MYSQLI_ASSOC);




$updateStr = "INSERT INTO `transactions` (g_price, gross_price_float) VALUES (('$applied' - ($applied - $totalArray[gross_total])), '$applied')";

$updateQuery = mysqli_query($connect,$updateStr);


echo "<center><h2>Applied a balance</h2></center>";

What I have tried:

I have tried to adjust the insert my last attempt is above.

Posted 28-Aug-24 2:43am

mcbain19

Updated 28-Aug-24 10:56am

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OriginalGriff · Answer 1 · 2024-08-28T19:40:00

Without your DB there isn't a lot we can do to help you, but two things do spring to mind.
1) Never concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Always use Parameterized queries instead.

When you concatenate strings, you cause problems because SQL receives commands like:

SQL

SELECT * FROM MyTable WHERE StreetAddress = 'Baker's Wood'

The quote the user added terminates the string as far as SQL is concerned and you get problems. But it could be worse. If I come along and type this instead: "x';DROP TABLE MyTable;--" Then SQL receives a very different command:

SQL

SELECT * FROM MyTable WHERE StreetAddress = 'x';DROP TABLE MyTable;--'

Which SQL sees as three separate commands:

SQL

SELECT * FROM MyTable WHERE StreetAddress = 'x';

A perfectly valid SELECT

SQL

DROP TABLE MyTable;

A perfectly valid "delete the table" command

SQL

--'

And everything else is a comment.
So it does: selects any matching rows, deletes the table from the DB, and ignores anything else.

So ALWAYS use parameterized queries! Or be prepared to restore your DB from backup frequently. You do take backups regularly, don't you?

2) That's some weird code: you appear to be using $applied as both an ID

PHP

$updateStr = "select... where ce.uid ='$applied'";

And as a mathematical value:

PHP

$updateStr = "INSERT INTO ... VALUES (('$applied' - ($applied - $totalArray[gross_total])), '$applied')";

But that's made even weirder because

Fragment

'$applied' - ($applied - $totalArray[gross_total])

will always be

$totalArray[gross_total]

anyway ...

I'd strongly suggest you start by thinking about what you are actually trying to do, and where things are stored! Then use the debugger to follow what your code is doing when it runs and watch what values to read / pass / end up with.