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XML provides the processing instruction as a means of passing
information to particular applications that may read the document.
A processing instruction starts with <? and ends with ?>.
Immediately following the <? is an XML name, called the target.
The target could perhaps be the name of the application for which the processing
instruction is intended, or it could be merely an identifier for this particular
processing instruction.
Examples include:
<?robots index="yes" follow="no"?>
In HTML, the tag:
<meta name="ROBOTS" content="NOINDEX, NOFOLLOW">
is used to tell search engines (and other robots) whether and how
they should index a page. the <?robots ... ?> XML processing
instruction has been proposed as an equivalent for XML documents (see http://www.atrus.org/writings/technical/robots_pi/spec-199912__/
)
<?php phpinfo(); ?>
In PHP, this processing instruction contains a call to a PHP function
which returns an HTML page full of information on how PHP was installed on
its web server.
In PHP, one often embeds a significant amount of PHP code inside a <?php
... ?> processing instruction.
<?mso-application progid="InfoPath.Document"?>
The Microsoft application InfoPath uses this
processing instruction to indicate that the XML file should be viewed within
the InfoPath client.
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