Description:
------------
When simplexml_load_string fails loading xml it returns FALSE. 

An error message should be available via error_get_last. This is not the case when certain encoding errors are encountered.

Reproduce code:
---------------
I'm using this code to gracefully fall back on encoding errors:

    $xml = simplexml_load_string($data);

    // character encoding warning- hack
    if ($xml === false) 
    {
        $error = error_get_last();
     
        // this is nasty- sometimes simplexml_load_string fails but doesn't raise an error
#        if (eregi('simplexml_load_string', $error['message']))
        {
            $xml = simplexml_load_string(utf8_encode($data));
        }
    }


Expected result:
----------------
This does _sometimes_ work (i.e. call the innermost, encoded version), but not always. For example, simplexml_load_string silently fails without error message when processing data from amazon like this:

http://ecs.amazonaws.com/onca/xml?Service=AWSECommerceService&Version=2007-12-24&Operation=ItemSearch&AssociateTag=cpuidle-20&AWSAccessKeyId=1CB01P12WQBRDNH10NR2&ItemPage=1&Keywords=Star+Wars%3A+Episode+1&

Actual result:
--------------
simplexml_load_string returns FALSE, but does not return error message.