Description:
------------
When an element of ArrayObject is incremented (++ operation), the object does not call its offsetSet method. I've tried overloading all public methods in ArrayObject: offsetGet is called, but not offsetSet or any other. Same with decrementing (--). This behaviour makes it impossible to handle all array modifications by overloading ArrayObject methods in derived class.

Reproduce code:
---------------
class myArray extends ArrayIterator
{
    public function offsetSet($index, $newval)
    {
        echo "offsetSet called\n";
        return parent::offsetSet($index, $newval);
    }
}

$a = new myArray(array('x' => 1));

echo "x += 1\n";
$a['x'] += 1; // calls offsetGet, then offsetSet with updated value
echo $a['x']."\n";

echo "x++\n";
$a['x']++; // only calls offsetGet
echo $a['x']."\n"; // but the value gets incremented


Expected result:
----------------
x += 1
offsetSet called
2
x++
offsetSet called
3


Actual result:
--------------
x += 1
offsetSet called
2
x++
3