When PHP parses a file, it looks for opening and closing tags,
which tell PHP to start and stop interpreting the code between
them. Parsing in this manner allows php to be embedded in all
sorts of different documents, as everything outside of a pair
of opening and closing tags is ignored by the PHP parser.
Most of the time you will see php embedded in HTML documents,
as in this example.
You can also use more advanced structures:
Example 10-1. Advanced escaping
<?php
if ($expression) {
?>
<strong>This is true.</strong>
<?php
} else {
?>
<strong>This is false.</strong>
<?php
}
?>
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This works as expected, because when PHP hits the ?> closing
tags, it simply starts outputting whatever it finds until it hits
another opening tag. The example given here is contrived, of
course, but for outputting large blocks of text, dropping out of
PHP parsing mode is generally more efficient than sending all of
the text through
echo() or
print().
There are four different pairs of opening and closing tags
which can be used in php. Two of those, <?php ?> and
<script language="php"> </script>, are always available.
The other two are short tags and ASP
style tags, and can be turned on and off from the php.ini
configuration file. As such, while some people find short tags
and ASP style tags convenient, they
are less portable, and generally not recommended.
Note:
Also note that if you are embedding PHP within XML or XHTML
you will need to use the <?php ?> tags to remain
compliant with standards.
Example 10-2. PHP Opening and Closing Tags
1. <?php echo 'if you want to serve XHTML or XML documents, do like this'; ?>
2. <script language="php">
echo 'some editors (like FrontPage) don\'t
like processing instructions';
</script>
3. <? echo 'this is the simplest, an SGML processing instruction'; ?>
<?= expression ?> This is a shortcut for "<? echo expression ?>"
4. <% echo 'You may optionally use ASP-style tags'; %>
<%= $variable; # This is a shortcut for "<% echo . . ." %>
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While the tags seen in examples one and two are both
always available, example one is the most commonly
used, and recommended, of the two.
Short tags (example three) are only available when they are
enabled via the short_open_tag
php.ini configuration file directive, or if php was configured
with the --enable-short-tags option.
Note:
If you are using PHP 3 you may also enable short tags via
the short_tags() function. This
is only available in PHP 3!
ASP style tags (example four) are only available when
they are enabled via the asp_tags php.ini
configuration file directive.
Note:
Support for ASP tags was added in 3.0.4.
Note:
Using short tags should be avoided when developing applications
or libraries that are meant for redistribution, or deployment on
PHP servers which are not under your control, because short tags
may not be supported on the target server. For portable,
redistributable code, be sure not to use short tags.
