The title contains over 350 pages of XML and PHP goodies. It walks you through the process of building a fully-functional XML-based content management system with PHP. And all the code used in the book is available to customers in a downloadalbe archive. As always, you can if you prefer. Introduction to XML Who here has heard of XML? Okay, just about everybody. So, what is XML? A significant portion of the group leans forward eagerly, wanting to learn more. The others either roll their eyes in anticipation of hype and half-formed theories, or cringe in fear of a long, dry history of markup languages. The essence of XML is in its name: Extensible Markup Language. Extensible XML is extensible. It lets you define your own tags, the order in which they occur, and how they should be processed or displayed. Another way to think about extensibility is to consider that XML allows all of us to extend our notion of what a document is: it can be a file that lives on a file server, or it can be a transient piece of data that flows between two computer systems as in the case of Web Services. Markup The most recognizable feature of XML is its tags, or elements to be more accurate. However, XML allows you to define your own set of tags. XML is a meta-language: a language that allows us to create or define other languages. For example, with XML we can create other languages, such as RSS, MathML a mathematical markup language , and even tools like XSLT. More on this later. Why Do We Need XML? Okay, we know what it is, but why do we need XML? We need it because HTML is specifically designed to describe documents for display in a Web browser, and not much else. HTML rarely if ever provides information about how the document is structured or what it means. Semantics is the study of meaning in language. Humans are much better at semantics than computers, because humans are really good at deriving meaning. From there, you could make the leap to other wild cats, then to house cats and maybe even dogs cats and dogs are both pets, after all. Needless to say, computers are really bad at this game, which is a shame, as many computing tasks require semantic skill. For example, a human can probably deduce tha