Chapter 5. Basic syntax
- Table of Contents
- Escaping from HTML
- Instruction separation
- Comments
Escaping from HTML
When PHP parses a file, it simply passes the text
of the file through until it encounters one of the special
tags which tell it to start interpreting the text as PHP code.
The parser then executes all the code it finds, up until it
runs into a PHP closing tag, which tells the parser to just
start passing the text through again. This is the mechanism
which allows you to embed PHP code inside HTML: everything
outside the PHP tags is left utterly alone, while everything
inside is parsed as code.
There are four sets of tags which can be used to
denote blocks of PHP code. Of these, only two (<?php.
. .?> and <script language="php">. . .</script>)
are always available; the others can be turned on or off from
the php.ini configuration file. While the short-form tags and ASP-style tags may be convenient,
they are not as portable as the longer versions. Also, if
you intend to embed PHP code in XML or XHTML, you will need
to use the <?php. . .?> form to conform to the XML.
The tags supported by PHP are:
Example 5-1. Ways of escaping from HTML
1. <?php
echo("if
you want to serve XHTML or XML documents, do like
this\n"); ?>
2. <? echo
("this is the simplest,
an SGML processing instruction\n");
?>
<?=
expression ?> This is a shortcut for
"<? echo
expression ?>"
3. <script
language="php">
echo
("some editors (like
FrontPage) don't
like
processing instructions");
</script>
4. <% echo ("You may optionally
use ASP-style tags"); %>
<%=
$variable; # This is a shortcut for "<% echo
. . ." %> |
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The first way, <?php. . .?>, is the preferred
method, as it allows the use of PHP in XML-conformant code
such as XHTML.
The second way is not available always. Short tags are
available only when they have been enabled. This can be done
via the short_tags() function (PHP 3 only), by enabling the short_open_tag configuration setting
in the PHP config file, or by compiling PHP with the --enable-short-tags
option to configure. Even if it is enabled by default in php.ini-dist, use of short tags are discouraged.
The fourth way is only available if ASP-style tags have
been enabled using the asp_tags configuration setting.
Note: Support for ASP-style tags was added in 3.0.4.
Note: Using short tags should be avoided when developing applications or libraries
that are meant for redistribution, or deployment on PHP
servers which are not under your control, because short
tags may not be supported on the target server. For portable,
redistributable code, be sure not to use short tags.
The closing tag for the block will include the immediately
trailing newline if one is present. Also, the closing tag
automatically implies a semicolon; you do not need to have
a semicolon terminating the last line of a PHP block. Closing
tag of a PHP block at the end of a file is optional.
PHP allows you to use structures like this:
Example 5-2. Advanced escaping
<?php
if ($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() or somesuch.
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