Why, when I'm adding two integers, does PHP return a bad result (6+7=1)?

Question

I have two variables:

$qty = 7;
$_POST['qty'] = 6;

var_dump($qty, $_POST['qty']); // both vars are integers
$_SESSION['qty'] = $qty + $_POST['qty'];
echo '='.$_SESSION['qty'];

This returns:

int(7) int(6) =1

(int)$qty, (int)$_POST['qty'] doesn't solve the problem.

What am I doing wrong?

Update:

... intval($qty) + intval($_POST['qty']);

not help.

And I notice one more detail. Problem apeear only when $_SESSION['qty'] >= 10:

$_SESSION['qty'] = $qty + $_POST['qty']; // $qty = 3, $_POST['qty'] = 6

Return good result ($_SESSION['qty'] = 9).

SOLVED

Thanks all for your unswers. But problem more not actual (it was a server problem). Anyway +1 to all.

Answer 1

Very odd indeed, I've never had this sort of problem. If you explicitly use the integer values in the arithmetic operation? (Not the same as casting to integer)

$_SESSION['qty'] = (intval($qty) + intval($_POST['qty']));

Have you tried using only your own variables, and leaving the POST-value out of the operation?

Answer 2

What you're doing with (int) is casting the variable, not converting it. You should use intval($var) instead.

See PHP: Integers.

Answer 3

What does this give you?

$localQty = 7;
$_POST['qty'] = 6;

$_SESSION['qty'] = ($localQty + $_POST['qty']);
var_dump($_SESSION['qty']);

Do you initialize your Session properly with session_start() at the beginning of your script?

Answer 4

Your $_SESSION is implicitly initialized as string somewhere

$_SESSION = 'blah';

$_SESSION['qty'] = 13;
var_dump($_SESSION['qty']); // gives "1"

Answer 5

I would print_r($_SESSION) to see what it is.