Page 614 - Beginning PHP 5.3

Part III: Using PHP in Practice
By using valid XML documents, applications that know nothing about each other can still communicate
effectively — they just have to exchange XML documents, and understand the meaning of the DTD
or XSD against which those documents are validated. This is one of the main features that make XML
so powerful.

Major Parts of an XML Document
Broadly speaking, a well - formed XML document may contain the following:

❑ An XML declaration at the top of the document, possibly including a character encoding
declaration. This declaration is a good idea, but it ’ s not mandatory. If no XML declaration is
given, version 1.0 is normally used. If no character encoding is specified, UTF - 8 is assumed.
For example:

< ?xml version=”1.0” encoding=”UTF-8”? >

❑ An optional DTD or an XSD, or a reference to one of these if they are stored externally. This
must appear before the document ’ s root element. For example, here ’ s a reference to an
external DTD:
< !DOCTYPE stockList SYSTEM “http://www.example.com/dtds/stockList.dtd” >

❑ All XML documents must contain one — and only one — root element. This element usually
contains one or more child elements, each of which may optionally have one or more attributes.
An element can contain other child elements or data between its beginning and ending tag, or it
may be empty

❑ XML documents may contain additional components such as processing instructions (PIs) that
provide machine instructions for particular applications; CDATA sections, which may contain
special characters that are not allowed in ordinary XML data; notations; comments; entity
references (aliases for entities such as special characters); text; and entities. You look at some of
these components later in the chapter

Here ’ s an enhanced version of the stockList example document used earlier that illustrates each of
these major parts. First the XML declaration:

< ?xml version=”1.0” encoding=”UTF-8”? >
Next is the reference to a DTD that defines the allowed elements and attributes in the document:

< !DOCTYPE stockList SYSTEM “http://www.example.com/dtds/stockList.dtd” >
Now the root element begins. Remember that there can be only one root element in an XML document:

< stockList >

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