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  <refentry id="function.header">
   <refnamediv>
    <refname>header</refname>
    <refpurpose>Send a raw HTTP header</refpurpose>
   </refnamediv>
   <refsect1>
    <title>Description</title>
     <methodsynopsis>
      <type>void</type><methodname>header</methodname>
      <methodparam><type>string</type><parameter>string</parameter></methodparam>
      <methodparam choice="opt"><type>bool</type><parameter>replace</parameter></methodparam>
      <methodparam choice="opt"><type>int</type><parameter>http_response_code</parameter></methodparam>
     </methodsynopsis>
    <para>
     <function>header</function> is used to send raw
     <acronym>HTTP</acronym> headers.  See the <ulink
     url="&url.rfc;2616">HTTP/1.1 specification</ulink> for more
     information on <acronym>HTTP</acronym> headers.
    </para>
    <para>
     The optional <parameter>replace</parameter> parameter indicates
     whether the header should replace a previous similar header, or
     add a second header of the same type.  By default it will replace,
     but if you pass in &false; as the second argument you can force
     multiple headers of the same type.  For example:
    </para>
    <para>
     <informalexample>
      <programlisting role="php">
<![CDATA[
<?php
header('WWW-Authenticate: Negotiate');
header('WWW-Authenticate: NTLM', false);
?>
]]>
      </programlisting>
     </informalexample>
    </para>
    <para>
     The second optional <parameter>http_response_code</parameter> force the
     HTTP response code to the specified value. (This parameter is available
     in PHP 4.3.0 and higher.)
    </para>
    <para>
     There are two special-case header calls.  The first is a header 
     that starts with the string "<literal>HTTP/</literal>" (case is not
     significant), which will be used to figure out the HTTP status
     code to send. For example, if you have configured Apache to
     use a PHP script to handle requests for missing files (using
     the <literal>ErrorDocument</literal> directive), you may want to
     make sure that your script generates the proper status code.
    </para>
    <para>
     <informalexample>
      <programlisting role="php">
<![CDATA[
<?php
header("HTTP/1.0 404 Not Found");
?>
]]>
     </programlisting>
    </informalexample>
   </para>
    <note>
     <para>
      The HTTP status header line will always be the first sent
      to the client, regardless of the actual <function>header</function> 
      call being the first or not. The status may be overridden 
      by calling <function>header</function> with a new status line
      at any time unless the HTTP headers have already been sent.
     </para>
    </note>
    <para>
     The second special case is the "Location:" header.  Not only does 
     it send this header back to the browser, but it also returns a 
     <literal>REDIRECT</literal> (302) status code to the browser unless 
     some <literal>3xx</literal> status code has already been set.
    </para>
    <para>
     <informalexample>
      <programlisting role="php">
<![CDATA[
<?php
header("Location: http://www.example.com/"); /* Redirect browser */

/* Make sure that code below does not get executed when we redirect. */
exit;
?>
]]>
      </programlisting>
     </informalexample>
    </para>
    <note>
     <para>
      HTTP/1.1 requires an absolute <acronym>URI</acronym> as argument to
      <ulink url="&spec.http1.1;-sec14.html#sec14.30">Location:</ulink>
      including the scheme, hostname and absolute path, but
      some clients accept relative URIs. You can usually use
      <literal>$_SERVER['HTTP_HOST']</literal>, <literal>$_SERVER['PHP_SELF']</literal>
      and <function>dirname</function> to make an absolute URI from a
      relative one yourself:
      <informalexample>
       <programlisting role="php">
<![CDATA[
<?php
header("Location: http://" . $_SERVER['HTTP_HOST']
                      . dirname($_SERVER['PHP_SELF'])
                      . "/" . $relative_url);
?>
]]>
       </programlisting>
      </informalexample>
     </para>
    </note>
    <para>
     PHP scripts often generate dynamic content that must not be cached
     by the client browser or any proxy caches between the server and the
     client browser. Many proxies and clients can be forced to disable
     caching with:
    </para>
    <para>
     <informalexample>
      <programlisting role="php">
<![CDATA[
<?php
// Date in the past
header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");

// always modified
header("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT");
 
// HTTP/1.1
header("Cache-Control: no-store, no-cache, must-revalidate");
header("Cache-Control: post-check=0, pre-check=0", false);

// HTTP/1.0
header("Pragma: no-cache");
?>
]]>
      </programlisting>
     </informalexample>
    </para>
    <note>
     <para>
      You may find that your pages aren't cached even if you don't
      output all of the headers above. There are a number of options
      that users may be able to set for their browser that change its
      default caching behavior. By sending the headers above, you should
      override any settings that may otherwise cause the output of your
      script to be cached.
     </para>
     <para>
      Additionally, <function>session_cache_limiter</function> and
      the <literal>session.cache_limiter</literal> configuration
      setting can be used to automatically generate the correct
      caching-related headers when sessions are being used.
     </para>
    </note>
    <para>
     Remember that <function>header</function> must be
     called before any actual output is sent, either by normal HTML
     tags, blank lines in a file, or from PHP. It is a very common
     error to read code with <function>include</function>, or
     <function>require</function>, functions, or another file access
     function, and have spaces or empty lines that are output before
     <function>header</function> is called. The same problem exists
     when using a single PHP/HTML file.
    </para>
    <para>
     <informalexample>
      <programlisting role="php">
<![CDATA[
<html>
<?php
/* This will give an error. Note the output
 * above, which is before the header() call */
header('Location: http://www.example.com/');
?>
]]>
      </programlisting>
     </informalexample>
    </para>    
    <note>
     <para>
      As of PHP 4, you can use output buffering to get around this problem,
      with the overhead of all of your output to the browser being buffered
      in the server until you send it. You can do this by calling
      <function>ob_start</function> and <function>ob_end_flush</function>
      in your script, or setting the <literal>output_buffering</literal>
      configuration directive on in your &php.ini; or
      server configuration files.
     </para>
    </note>
    <para>
     If you want the user to be prompted to save the data you are
     sending, such as a generated PDF file, you can use the <ulink
     url="&url.rfc;2183">Content-Disposition</ulink> header to
     supply a recommended filename and force the browser to display the
     save dialog.
    </para>
    <para>
     <informalexample>
      <programlisting role="php">
<![CDATA[
<?php
// We'll be outputting a PDF
header('Content-type: application/pdf');

// It will be called downloaded.pdf
header('Content-Disposition: attachment; filename="downloaded.pdf"');

// The PDF source is in original.pdf
readfile('original.pdf');
?>
]]>
      </programlisting>
     </informalexample>
    </para>
    <note>
     <para>
      There is a bug in Microsoft Internet Explorer 4.01 that prevents
      this from working. There is no workaround. There is also a bug
      in Microsoft Internet Explorer 5.5 that interferes with this,
      which can be resolved by upgrading to Service Pack 2 or later.
     </para>
    </note>
    <note>
     <simpara>
      If <link linkend="ini.safe-mode">safe mode</link> is enabled the
      uid of the script is added to the <literal>realm</literal> part
      of the <literal>WWW-Authenticate</literal> header if you set
      this header (used for HTTP Authentication).
     </simpara>
    </note>
    <para>
     See also <function>headers_sent</function>,
     <function>setcookie</function>, and the section on 
     <link linkend="features.http-auth">HTTP authentication</link>.
    </para>
   </refsect1>
  </refentry>

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