One possibility that let's you still trap the error in your error log:
$er = error_reporting(0);
$xml = simplexml_load_string('test');
error_reporting($er);
if($xml === false)
{
$error = error_get_last();
error_log(print_r($error,1));
echo "Could not parse XML";
}
else
{
echo "It worked:<br />\n<pre>".print_r($xml,1)."</pre>";
}