write xml file to disk

I’d like to automate the writing of a xml file to a disk on my server. The data I want to include in the xml file is within the custom fields of a post. I am using the plug-in ‘get custom field values’ to show this information on my blog but would like to automatically update the xml file when additional posts are created. Is there a plug in that can do this for me or can someone point me in the right direction to script this into a template file? I’m new to wp and php but can give it a go! thanks.