Description:
------------
If I extend SimpleXmlElement and add a __toString() method to it, that method is not called when I cast my object using (string)$obj.
It is used however on implicit casts (e.g echo $sxmlObj)

Reproduce code:
---------------
class MyXml extends SimpleXMLElement 
{
    public function __toString()
    {
        return $this->asXML();
    }

    public function export()
    {
        return (string)$this;
    }
}
$xml = new MyXml("<xml><some>xml</some></xml>");
echo $xml; // 1
$a = (string)$xml;
echo $a; // 2
echo $xml->export(); // 3

Expected result:
----------------
1 :
xml returned

2 :
xml returned

3:
xml returned

Actual result:
--------------
1 :
OK

2:
nothing is returned

3:
nothing is returned