Description:
------------
implicit call to __toString() for the extended class does not work properly an empty value is returned (even without the try catch block nothing is returned)

Reproduce code:
---------------
<?php
class XML extends SimpleXMLElement 
{
	public function __toString()
	{
		try
		{
			$xml = new DOMDocument('1.0', 'utf-8');
			$xml->formatOutput = TRUE;
			$xml->preserveWhiteSpace = FALSE;
			$xml->loadXML($this->asXML());
			$retval = $xml->saveXML();
		}
		catch(Exception $e)
		{
			$retval = '';
		}
		return $retval;
	}
}
$xml = new XML('<foo><bar/></foo>');

echo '|' . $xml . "|\n";
echo '|' . (string) $xml . "|\n";
echo '|' . $xml->__toString() . "|\n";


Expected result:
----------------
|<?xml version="1.0"?>
<foo>
  <bar/>
</foo>
|
|<?xml version="1.0"?>
<foo>
  <bar/>
</foo>
|
|<?xml version="1.0"?>
<foo>
  <bar/>
</foo>
|


Actual result:
--------------
||
||
|<?xml version="1.0"?>
<foo>
  <bar/>
</foo>
|