$rss = "http://www.doviz.com/servis/rss/doviz/serbest-piyasa";

$rss_oku = simplexml_load_file($rss);

$dolar = $rss_oku->channel->item->description; 
$euro = $rss_oku->channel->item[1]->description;
$sterlin = $rss_oku->channel->item[2]->description;