check valid xml and then response time of rss-feed

I'm reading a rss feed and check I'm want to know how much time its taken to call

    $homepage = file_get_contents('http://www.forbes.com/news/index.xml');

for this I'm using

 <?php

$start = microtime(true);
$homepage = file_get_contents('http://www.fohgggrbes.com/news/index.xml');
 $end = microtime(true);
 $dur=$end-$start;
 echo $dur;
  $xml = simplexml_load_string($homepage,'SimpleXMLElement', LIBXML_NOCDATA);
  echo '<pre>';
?>

but my requirement is first I check the file is valid xml ,if is it ,then its show response time

 $homepage = file_get_contents('http://www.forbes.com/news/index.xml');

or if the XML file is not valid then shows error message. I want to use try and catch exceptions.