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CHAPTER 4 XPATH, XPOINTER, XINCLUDE, AND THE FUTURE 157 As expected, the element is not found, resulting in an error and the following document: Element not found XInclude Summary XInclude can be a useful technology to employ. Documents can be smaller in size as well as reused. You can overcome many of the issues with external entities using this method as well as the added ability to fall back to another case in the event of a failure. You can now handle fatal errors, preventing the complete stoppage of processing. In addition, you don t have to load external resources during initial parsing. If the resources are not needed at the time, the xincludes do not need to be processed, which not only keeps the document smaller but also reduces the processing time. Examining the Future of XML Some new technologies are on the horizon in the XML realm. Though not yet standards, these technologies are already being used in many commercialized products. The following sections cover these technologies; I ll also provide an overview of XLink, which although not a new technology is one that does not have as widespread use or support as the technologies already covered. Introducing XLink Although XLink has been a W3C standard for many years now since June 2001 (http:// www.w3.org/TR/xlink) this chapter will not provide an in-depth examination of this technology. It is primarily a UI-based technology, and currently neither PHP nor libxml has native support for XLink. You can create documents containing XLink elements by using extensions such as the DOM extension (which will be covered in Chapter 6), but no XLink processing abilities are offered. You may find some newer browsers beginning to support XLink, but unless you are within a controlled environment, it is not recommended to use XLink for a public site. XLink is for creating and describing links between resources. In terms of HTML, it would be the equivalent to the anchor tag, , on steroids. XLink lives within the http://www.w3.org/1999/xlink namespace. For the purposes of this section, the prefix xlink will be associated with this namespace. XLink allows any element to become a link. This is a big difference from HTML, where the only link is the anchor element. Listing 4-8 illustrates a sample document using XLink.

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