In an excerpt from the APress book "Beginning Google Maps Applications with PHP and AJAX: From Novice to Professional", this entry talks about caching the responses from a remote data source into the $_SESSION superglobal during development.
Why spend time setting up a sophisticated cache for the whole responses while you develop your XPath expressions and regexes? Especially if you ultimately will only be caching the results of these actions?
One solution is to simply store the responses in the PHP variable $_SESSION, and then read them out again on the next viewing. To make it even easier, you can just wrap your remote calls in some code that checks it for you.
Their "weapon of choice" is cURL using a wrapper class (specifically this one from Sean Hunter) around it to amke the connection and cache the results. They also include the source of the script as an example.