When you google to get a solution to retrieve all the or single tag xml values/attributes you may get approx 10,400,000 results.

Common solution will be to create a module, bring down your own function, bla-bla bla.......

The simplest and statistical way to get is using inbuild drupal functions.

1.  $getXML = drupal_http_request( $url );  //$url is the url to get the xml

2.  To retrieve only single tag.