Here is a simple code

<?
header("content-type: text/plain");

$num = -5;
echo "$num\n";
$num++;
echo "$num\n";
$num--;
echo "$num\n";
echo "----------\n";

$num = "-5";
echo "$num\n";
$num++;
echo "$num\n";
$num--;
echo "$num\n";
?>

And here results

-5
-4
-5
----------
-5
-6 <-- ?????????????????
-7

It looks like "++" operator do not work properly on variables enclosed in double-quotes.
Other operators (ie $num = $num +1) woks properly.