On SitePoint.com today there's a new post from Craig Buckler showing you how to create a simple XML to JSON proxy server in PHP with a SimpleXML object at its heart.

Unless you're new to this web development lark, you'll know the 'X' in 'AJAX' stands for XML - eXtensible Markup Language. But you’re probably not using XML. If you are, you'd probably prefer not to. All the cool kids are using JSON or JSON-P: it has a smaller payload, is easier to use and faster to process. [...] Fortunately, there are a couple of solutions which allow you to retain the benefits of XML data interchange but provide the ease of JSON in JavaScript. In this article, we’re going to create an XML to JSON proxy server in PHP.

Of course, this will only work with well-formatted XML documents, but it's a quick little hack that pulls in the XML data with a curl request and parses it via SimpleXML and uses json_encode to push it back out as JSON.