Programming PHP, 2nd Edition

Lightning Guide to XML

Most XML consists of elements (like HTML tags), entities, and regular data. For example:

<book isbn="1-56592-610-2">
  <title>Programming PHP</title>
  <authors>
    <author>Rasmus Lerdorf</author>
    <author>Kevin Tatroe</author>
    <author>Peter MacIntyre</author>
  </authors>
</book>

In HTML, you often have an open tag without a close tag. The most common example of this is:

<br>

In XML, that is illegal. XML requires that every open tag be closed. For tags that don’t enclose anything, such as the line break <br>, XML adds this syntax:

<br />

Tags can be nested but cannot overlap. For example, this is valid:

<book><title>Programming PHP</title></book>

but this is not valid, because the book and title tags overlap:

<book><title>Programming PHP</book></title>

XML also requires that the document begin with a processing instruction that identifies the version of XML being used (and possibly other things, such as the text encoding used). For example:

<?xml version="1.0" ?>

The final requirement of a well-formed XML document is that there be only one element at the top level of the file. For example, this is well formed:

<?xml version="1.0" ?>
<library>
  <title>Programming PHP</title>
  <title>Programming Perl</title>
  <title>Programming C#</title>
</library>

but this is not well formed, as there are three elements at the top level of the file:

<?xml version="1.0" ?>
<title>Programming PHP</title>
<title>Programming Perl</title>
<title>Programming C#</title>

XML documents generally are not ...