I'd start with code like this:
$input = file_get_contents($url) or die('Could not access file: $url');
$xmlobj = simplexml_load_string($input);
This will get the XML document at $url (the rss feed), convert it into a PHP object, and dump out the resulting object so you can see what the structure of the object looks like.
You can use normal object notation ($xmlobj->attribute) to access parts of the rss feed object after that. Mostly these will take the form of php arrays that you can use a foreach loop to process.
Hope this helps!