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!