Problems fopen like parameter

I am opening a rss file using fopen.

If I use direct value, it can be opened, but I use a paremeter, I  
obtain an error.


This works:

   ...
   xml_set_character_data_handler($xml_parser, "characterData");
   $fp = fopen("http://www.arteglobal.net/news.xml","r") or die("Error  
reading RSS data.");
   while ($data = fread($fp, 4096))
     {
     xml_parse($xml_parser, $data, feof($fp))
     ...

But this one not (Error reading RSS data):

   ...
   $url_rss = "http://www.arteglobal.net/news.xml";
   ...
   xml_set_character_data_handler($xml_parser, "characterData");
   $fp = fopen($url_rss,"r") or die("Error reading RSS data.");
   while ($data = fread($fp, 4096))
     {
     xml_parse($xml_parser, $data, feof($fp))
     ...


Any idea about what is happening?.


Thank you!