Relating XML to HTML
In spite of the growing popularity of XML for storing and exchanging data of nearly any kind imaginable, XML is not well suited to act as a direct replacement for some of its defined subsets or sublanguages, like HTML. This is because XML defines only a standard for structuring dataXML itself fails (indeed, by design) to provide any standard for how XML data in the general case should be rendered or displayed to the user.
Such concerns, particularly in the case of the World Wide Web and the documents that it contains, are the domain of XML-compliant document type definitions such as Hypertext Markup Language (HTML) or Extensible Hypertext Markup Language (XHTML). Displaying and rendering standards like XHTML govern the ways in which the data and tags that form the structure of compliant XML documents are actually rendered onscreen for readers or World Wide Web perusers.
XSLT tools in PHP are designed to provide Web designers with the tools necessary to transform well-formatted XML documents with informative but renderer-agnostic tags into HTML or XHTML documents that contain the same text data, only structured for transient display on the World Wide Web, rather than for exchange and storage.