php-doc-en/language/basic-syntax.xml

<?xml version="1.0" encoding="utf-8"?>
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 <chapter xml:id="language.basic-syntax" xmlns="http://docbook.org/ns/docbook">
  <title>Basic syntax</title>
  <sect1 xml:id="language.basic-syntax.phptags">
   <title>PHP tags</title>
   <para>
    When PHP parses a file, it looks for opening and closing tags, which are
    <literal>&lt;?php</literal> and <literal>?&gt;</literal> 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.
   </para>

   <para>
    PHP includes a short echo tag <literal>&lt;?=</literal> which is a
    short-hand to the more verbose <code>&lt;?php echo</code>.
   </para>

   <para>
    <example>
     <title>PHP Opening and Closing Tags</title>
     <programlisting role="php">
<![CDATA[
1.  <?php echo 'if you want to serve PHP code in XHTML or XML documents,
                use these tags'; ?>

2.  You can use the short echo tag to <?= 'print this string' ?>.
    It's equivalent to <?php echo 'print this string' ?>.

3.  <? echo 'this code is within short tags, but will only work '.
            'if short_open_tag is enabled'; ?>
]]>
     </programlisting>
    </example>
   </para>

   <para>
    Short tags (example three) are available by default but can be disabled
    either via the <link linkend="ini.short-open-tag">short_open_tag</link>
    &php.ini; configuration file directive, or are disabled by default if
    PHP is built with the <option>--disable-short-tags</option> configuration.
   </para>
   <para>
    <note>
     <para>
      As short tags can be disabled it is recommended to only use the normal
      tags (<code>&lt;?php ?&gt;</code> and <code>&lt;?= ?&gt;</code>) to
      maximise compatibility.
     </para>
    </note>
   </para>

   <para>
    If a file contains only PHP code, it is preferable to omit the PHP closing tag
    at the end of the file.  This prevents accidental whitespace or new lines
    being added after the PHP closing tag, which may cause unwanted effects
    because PHP will start output buffering when there is no intention from
    the programmer to send any output at that point in the script.
    <informalexample>
     <programlisting role="php">
<![CDATA[
<?php
echo "Hello world";

// ... more code

echo "Last statement";

// the script ends here with no PHP closing tag
]]>
     </programlisting>
    </informalexample>
   </para>
  </sect1>

  <sect1 xml:id="language.basic-syntax.phpmode">
   <title>Escaping from HTML</title>
   <para>
    Everything outside of a pair of opening and closing tags is ignored by the
    PHP parser which allows PHP files to have mixed content.  This allows PHP
    to be embedded in HTML documents, for example to create templates.
    <informalexample>
     <programlisting role="php">
<![CDATA[
<p>This is going to be ignored by PHP and displayed by the browser.</p>
<?php echo 'While this is going to be parsed.'; ?>
<p>This will also be ignored by PHP and displayed by the browser.</p>
]]>
     </programlisting>
    </informalexample>
    This works as expected, because when the PHP interpreter hits the ?&gt; closing
    tags, it simply starts outputting whatever it finds (except for the
    immediately following newline - see
    <link linkend="language.basic-syntax.instruction-separation">instruction separation</link>)
    until it hits another opening tag unless in the middle of a conditional
    statement in which case the interpreter will determine the outcome of the
    conditional before making a decision of what to skip over.
    See the next example.
   </para>
   <para>
    Using structures with conditions
    <example>
     <title>Advanced escaping using conditions</title>
     <programlisting role="php">
<![CDATA[
<?php if ($expression == true): ?>
  This will show if the expression is true.
<?php else: ?>
  Otherwise this will show.
<?php endif; ?>
]]>
     </programlisting>
    </example>
    In this example PHP will skip the blocks where the condition is not met, even
    though they are outside of the PHP open/close tags; PHP skips them according
    to the condition since the PHP interpreter will jump over blocks contained
    within a condition that is not met.
   </para>
   <para>
    For outputting large blocks of text, dropping out of PHP parsing mode is
    generally more efficient than sending all of the text through
    <function>echo</function> or <function>print</function>.
   </para>
   <para>
    <note>
     <para>
      If PHP is embeded within XML or XHTML the normal PHP
      <code>&lt;?php ?&gt;</code> must be used to remain compliant
      with the standards.
     </para>
    </note>
   </para>
  </sect1>

  <sect1 xml:id="language.basic-syntax.instruction-separation">
   <title>Instruction separation</title>
   <para>
    As in C or Perl, PHP requires instructions to be terminated
    with a semicolon at the end of each statement. The closing tag
    of a block of PHP code automatically implies a semicolon; you
    do not need to have a semicolon terminating the last line of a
    PHP block. The closing tag for the block will include the immediately
    trailing newline if one is present.
   </para>

   <para>
    <example>
     <title>Example showing the closing tag encompassing the trailing newline</title>
     <programlisting role="php">
<![CDATA[
<?php echo "Some text"; ?>
No newline
<?= "But newline now" ?>
]]>
     </programlisting>
     &example.outputs;
     <screen>
<![CDATA[
Some textNo newline
But newline now
]]>
     </screen>
    </example>
   </para>

   <para>
    Examples of entering and exiting the PHP parser:

    <informalexample>
     <programlisting role="php">
<![CDATA[
<?php
    echo 'This is a test';
?>

<?php echo 'This is a test' ?>

<?php echo 'We omitted the last closing tag';
]]>
     </programlisting>
    </informalexample>
    <note>
     <para>
      The closing tag of a PHP block at the end of a file is optional,
      and in some cases omitting it is helpful when using <function>include</function>
      or <function>require</function>, so unwanted whitespace will
      not occur at the end of files, and you will still be able to add
      headers to the response later. It is also handy if you use output
      buffering, and would not like to see added unwanted whitespace
      at the end of the parts generated by the included files.
     </para>
    </note>
   </para>
  </sect1>
  <sect1 xml:id="language.basic-syntax.comments">
   <title>Comments</title>
   <para>
    PHP supports 'C', 'C++' and Unix shell-style (Perl style) comments. For example:

    <informalexample>
     <programlisting role="php">
<![CDATA[
<?php
    echo 'This is a test'; // This is a one-line c++ style comment
    /* This is a multi line comment
       yet another line of comment */
    echo 'This is yet another test';
    echo 'One Final Test'; # This is a one-line shell-style comment
?>
]]>
     </programlisting>
    </informalexample>
   </para>
   <simpara>
    The "one-line" comment styles only comment to the end of
    the line or the current block of PHP code, whichever comes first.
    This means that HTML code after <literal>// ... ?&gt;</literal>
    or <literal># ...  ?&gt;</literal> WILL be printed:
    ?&gt; breaks out of PHP mode and returns to HTML mode, and
    <literal>//</literal> or <literal>#</literal> cannot influence that.
   </simpara>
   <para>
    <informalexample>
     <programlisting role="php">
<![CDATA[
<h1>This is an <?php # echo 'simple';?> example</h1>
<p>The header above will say 'This is an  example'.</p>
]]>
     </programlisting>
    </informalexample>
   </para>
   <simpara>
    'C' style comments end at the first <literal>*/</literal> encountered.
    Make sure you don't nest 'C' style comments.  It is easy to make this
    mistake if you are trying to comment out a large block of code.
   </simpara>
   <para>
    <informalexample>
     <programlisting role="php">
<![CDATA[
<?php
 /*
    echo 'This is a test'; /* This comment will cause a problem */
 */
?>
]]>
     </programlisting>
    </informalexample>
   </para>
  </sect1>
 </chapter>

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