As long as your site is built in php (and you claim to be a freelancer web designer and developer with a large portfolio in your site, don't you?), I presume that you have a DB (most sure MySQL) in back-end, haven't you? If so, why do you need an XML data store?
Regarding your question: yes, it is possible to use XML as data store (but it is not recommended). The access from a web document to and fro an XML file is to be made in several ways, AJAX is the most popular.