A Really, Really, Really Good Introduction to XML

هم اکنون نیازمند یاریتان هستیم.

In this chapter, we'll cover the basics of XML — essentially, most of the information you'll need to know to get a handle on this exciting technology. After we're done exploring some terminology and examples, we'll jump right in and start working with XML documents. Then, we'll spend some time starting the project we'll develop through the course of this book: building an XML-powered content management system.

This excerpt is taken from No Nonsense XML Web Development with PHP, SitePoint's new release, by Thomas Myer, which was designed to help you start using XML to build intelligent 'Future-Proof' PHP applications today.

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.

To find out more about "No Nonsense XML Web Development with PHP", visit the book's information page, or review the contents of the entire publication. As always, you can download this excerpt as a PDF if you prefer.

Chapter 1. Introduction to XML

Who here has heard of XML? Okay, just about everybody. If ever there were a candidate for "Most Hyped Technology" during the late 90s and the current decade, it's XML (though Java would be a close contender for the title).

Whenever I talk about XML with developers, designers, technical writers, or other Web professionals, the most common question I'm asked is, "What's the big deal?" In this book, I'll explain exactly what the big deal is—how XML can be used to make your Web applications smarter, more versatile, and more powerful. I'll try to stay away from the grandstanding hoopla that has characterized much of the discussion of XML; instead, I'll give you the background and know-how you'll need to make XML a part of your professional skillset.

What is XML?

So, what is XML? Whenever a group of people asks this question, I always look at the individuals' body language. 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. As a result, I've learned to keep my explanation brief.

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). In fact, the elements you'll create in XML will be very similar to the elements you've already been creating in your HTML documents. However, XML allows you to define your own set of tags.

Language

XML is a language that's very similar to HTML. It's much more flexible than HTML because it allows you to create your own custom tags. However, it's important to realize that XML is not just a language. 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. It becomes cumbersome if you want to display documents in a mobile device or do anything that's even slightly complicated, such as translating the content from German to English. HTML's sole purpose is to allow anyone to quickly create Web documents that can be shared with other people. XML, on the other hand, isn't just suited to the Web—it can be used in a variety of different contexts, some of which may not have anything to do with humans interacting with content (for example, Web Services use XML to send requests and responses back and forth).

HTML rarely (if ever) provides information about how the document is structured or what it means. In layman's terms, HTML is a presentation language, whereas XML is a data-description language.

For example, if you were to go to any ecommerce Website and download a product listing, you'd probably get something like this:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" 
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 
<html xmlns="http://www.w3.org/1999/xhtml"> 
<head> 
<title>ABC Products</title> 
<meta http-equiv="Content-Type" 
    content="text/html; charset=iso-8859-1" /> 
</head> 
<body> 
<h1>ABC Products</h1> 
<h2>Product One</h2> 
<p>Product One is an exciting new widget that will simplify your 
  life.</p> 
<p><b>Cost: $19.95</b></p> 
<p><b>Shipping: $2.95</b></p> 
<h2>Product Two</h2> 
… 
<h3>Product Three</h3> 
<p><i>Cost: $24.95</i></p> 
<p>This is such a terrific widget that you will most certainly 
  want to buy one for your home and another one for your 
  office!</p> 
… 
</body> 
</html>

Take a good look at this—admittedly simple—code sample from a computer's perspective. A human can certainly read this document and make the necessary semantic leaps to understand it, but a computer couldn't.