reading data from xml feed using simplexml

Im using simplexml to bring in a data feed, and now i want to put that into working varables to use in my php doc.

Following the php.net guides on simplexml ive arrived at

<?php    $xml = simplexml_load_file('f1_feed.xml');    $xml = new SimpleXMLElement($xmlstr);    echo $xml->response->williamhill->class->type->market[0]->name;?>

but i keep getting a blank page, have i completely missed to point of how to parse the xml and put it into a working var ?

(feed is local for development)