PHP - Get random image from xml file

I want to display a random image from the Unsplash.com RSS feed (http://ift.tt/1vTYJ0o) using PHP. I've read about how to this, but I can't seem to quite understand it. I think the easiest way to do this is would be with SimpleXML. Then I'd like to use the image as a background-image for my website. Can anyone help me out with this? Thanks!