Как сделать вычисление $cost из баланса?

Question

[Sigar49]

Code:

<?php
$corp = $_GET['corp'];
$cost = $_GET['cost'];
$card = $_POST['card'];
$code = $_POST['code'];

if (mb_strlen($card) < 4) {
	header('location: not-found-card.php');
}
if (mb_strlen($code) < 4) {
	header('location: not-found-card.php');
}

$mysql = new mysqli('localhost', 'root', '', 'db');
$result = $mysql->query("SELECT * FROM `reg` WHERE `card`='$card' AND `code`='$code'");
$user = $result->fetch_assoc();
if (count($user) == 0) {
	header('location: not-found-card.php');
}

$mysql->query("SELECT `balance` FROM `reg` WHERE `card`='$card' AND `code`='$code'");
//ТУТ НУЖНО СОЗДАТЬ ВЫЧИСЛЕНИЕ ДЕНЕГ ИЗ БАЛАНСА ($cost - balance).
$mysql->close();

header('location: /);

?>

Если можно скиньте ответ кодом.

Answer 1

[Slava Rozhnev]

$stmt = $mysqli->prepare("UPDATE req 
	SET balance = balance - ? 
	WHERE card =? AND code = ? ;");
$stmt->bind_param("sss", $cost, $card, $code);

// set parameters and execute
$cost = $_GET['cost'];
$card = $_POST['card'];
$code = $_POST['code'];

$stmt->execute();

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