I've created an API and I want to enable caching. The API is written in PHP, is RESTful and delivers the results in XML. I've googled PHP and caching API and have only come across ways to get past social site APIs, not how to cache your own. The API is accessed via desktop client and browser. I'm using the DOMDocument class and returning text, plus image URLs, all of which could benefit from caching.

Any suggestions on where to start? Developers access the API with tokens, so I'd like to enable hourly limiting, etc.