simpleXML to dynamically as add a feed

I need to add a foreighn blog feed dynamically. Using Simple XML, I get the following error:

Warning: simplexml_load_file( failed to open stream: HTTP request failed! HTTP/1.1 403 Forbidden in /home1/pmacarter/public_html/wp-content/themes/wp-strapthirteen/directory.php on line 77

Lines 76-77 are as follows:
$myurl = $feedURL; // url to parse
$rss = simplexml_load_file($myurl); // the XML parser

Why does this generate a 403 error??