Description:
------------
When extending SimpleXMLElement the __get() and __set() methods of the extending class do not get invoked. 
__call however works as expected.

Reproduce code:
---------------
<?php

class TestSimpleXMLElement extends SimpleXMLElement {
    function __set($name, $value) { echo "__set $name\n"; }
    function __get($name) { echo "__get $name\n"; }
    function __call($name, $parameters) { echo "__call $name\n"; }
}

$element = new TestSimpleXMLElement('<root/>');
// __set
$element->child1 = 1;
// __get
$element->child2;
// __call
$element->method();

?>

Expected result:
----------------
__set child1
__get child2
__call method

Actual result:
--------------
__call method