I wanted to play with YouTube's API to see if I can come up with something, but I can't find a good step by step tutorial on using XML with PHP, and I need one that uses curl_init() instead of fopen() (host restrictions).
Are you using PHP4 or PHP5? 5 is much easier to work with XML, but it's possible with 4 as well - with 5 you can probably use the built in stuff that php has. Check out here for php5 and XML.
If you're using 4, it's a bit more of a pain since the best option is probably to use a third person library.
You can check out this library for php4 xml, and the php4 documentation for using its class library is here.
As for using curl vs. fopen, I assume your host won't let you use fopen for remote urls. If that is the case, the easiest method is probably to use curl to get the file, and then use fopen locally to open it. Something like
PHP:
$ch = curl_init($url); // open curl
$fp = fopen(path/to/new/file); // create a file to save the information in
curl_setopt($ch, CURLOPT_FILE, $fp); // save info to file
curl_exec($ch); // execute
curl_close($ch);
Now you can open the file and do any xml manipulations you want.
I'm sure there is a way to do this directly without the file save, but I haven't used curl in awhile so that's the best quick answer I can come up with.