Description:
------------
I would expect casting to a string to have the same result as calling the 
__toString() function on extending a SimpleXMLElement object. 
I've tested this against

PHP 5.3.3-7+squeeze13 with Suhosin-Patch (cli) (built: Jun 10 2012 07:31:32) 
Copyright (c) 1997-2009 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2010 Zend Technologies
    with XCache v1.3.0, Copyright (c) 2005-2009, by mOo
    with Xdebug v2.1.2, Copyright (c) 2002-2011, by Derick Rethans
    with Suhosin v0.9.32.1, Copyright (c) 2007-2010, by SektionEins GmbH

and 

PHP 5.4.4 (cli) (built: Jun 14 2012 11:50:02) 
Copyright (c) 1997-2012 The PHP Group
Zend Engine v2.4.0, Copyright (c) 1998-2012 Zend Technologies

and 


PHP 5.3.14 (cli) (built: Jun 14 2012 11:49:39) 
Copyright (c) 1997-2012 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2012 Zend Technologies


Test script:
---------------
<?php
class UberSimpleXML extends SimpleXMLElement {
	public function __toString() {
		return 'stringification';
	}
}

$xml = new UberSimpleXML('<xml/>');
$xml->tag = "you're it!";

echo "\nno cast\n";
echo $xml;
echo "\ncast\n";
echo (string) $xml;
echo "\nexplicit call\n";
echo $xml->__toString();
echo "\n";
var_dump((string) $xml);
var_dump($xml->__toString());


Expected result:
----------------
no cast
stringification
cast

explicit call
stringification
string(0) ""
string(15) "stringification"


Actual result:
--------------
no cast
stringification
cast
stringification
explicit call
stringification
string(15) "stringification"
string(15) "stringification"