PHP Arithmetic

When I use:
$grandtotal = ($_SESSION['value'] + $_SESSION['cost'] * 100);
or
$grandtotal = $_SESSION['value'] + $_SESSION['cost'] * 100;
I get 122.50

Why doesn't it give me 12250?

Order of operations.. plus the values contained in your session variables might be wrong.

$grandtotal = ($_SESSION['value'] + $_SESSION['cost']) * 100;