Programming PHP

In HTML, you often have an open tag without a close tag. The most common example
of this is:
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 , XML adds this syntax:
Tags can be nested but cannot overlap. For example, this is valid:
Programming PHP
but this is not valid, because the book and title tags overlap:
Programming PHP
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:
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:
Programming PHP
Programming Perl
Programming C#
but this is not well formed, as there are three elements at the top level of the file:
Programming PHP
Programming Perl
Programming C#
XML documents generally are not completely ad hoc. The specific tags, attributes,
and entities in an XML document, and the rules governing how they nest, comprise
the structure of the document. There are two ways to write down this structure: the
Document Type Definition (DTD) and the Schema. DTDs and Schemas are used to
validate documents; that is, to ensure that they follow the rules for their type of
document.
Most XML documents don’t include a DTD. Many identify the DTD as an external
with a line that gives the name and location (file or URL) of the DTD:
Sometimes it’s convenient to encapsulate one XML document in another. For example,
an XML document representing a mail message might have an attachment element
that surrounds an attached file. If the attached file is XML, it’s a nested XML
This is the Title of the Book, eMatter Edition
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