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                                    header

   (PHP 3, PHP 4 )
   header -- Send a raw HTTP header

Description

   void header ( string string [, bool replace [, int
   http_response_code]])

   header() is used to send raw HTTP headers. See the HTTP/1.1
   specification for more information on HTTP headers.

   The optional replace 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:

   <?php
   header('WWW-Authenticate: Negotiate');
   header('WWW-Authenticate: NTLM', false);
   ?>

   The second optional http_response_code force the HTTP response code to
   the specified value. (This parameter is available in PHP 4.3.0 and
   higher.)

   There are two special-case header calls. The first is a header that
   starts with the string "HTTP/" (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 ErrorDocument directive), you may want to
   make sure that your script generates the proper status code.

   <?php
   header("HTTP/1.0 404 Not Found");
   ?>

     Note: The HTTP status header line will always be the first sent to
     the client, regardless of the actual header() call being the first
     or not. The status may be overridden by calling header() with a new
     status line at any time unless the HTTP headers have already been
     sent.

     Note: In PHP 3, this only works when PHP is compiled as an Apache
     module. You can achieve the same effect using the Status header.

   <?php
   header("Status: 404 Not Found");
   ?>

   The second special case is the "Location:" header. Not only does it
   send this header back to the browser, but it also returns a REDIRECT
   (302) status code to the browser unless some 3xx status code has
   already been set.

   <?php
   header("Location: http://www.example.com/"); /* Redirect browser */
   /* Make sure that code below does not get executed when we redirect.
   */
   exit;
   ?>

     Note: HTTP/1.1 requires an absolute URI as argument to Location:
     including the scheme, hostname and absolute path, but some clients
     accept relative URIs. You can usually use $_SERVER['HTTP_HOST'],
     $_SERVER['PHP_SELF'] and dirname() to make an absolute URI from a
     relative one yourself:

   <?php
   header("Location: http://" . $_SERVER['HTTP_HOST']
                         . dirname($_SERVER['PHP_SELF'])
                         . "/" . $relative_url);
   ?>

   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:

   <?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");
   ?>

     Note: 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.

     Additionally, session_cache_limiter() and the session.cache_limiter
     configuration setting can be used to automatically generate the
     correct caching-related headers when sessions are being used.

   Remember that header() 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 include(), or require(),
   functions, or another file access function, and have spaces or empty
   lines that are output before header() is called. The same problem
   exists when using a single PHP/HTML file.

   <html>
   <?php
   /* This will give an error. Note the output
   * above, which is before the header() call */
   header('Location: http://www.example.com/');
   ?>

     Note: In 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 ob_start() and ob_end_flush() in your script, or setting
     the output_buffering configuration directive on in your php.ini or
     server configuration files.

   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 Content-Disposition
   header to supply a recommended filename and force the browser to
   display the save dialog.

   <?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');
   ?>

     Note: 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.

     Note: If safe mode is enabled the uid of the script is added to the
     realm part of the WWW-Authenticate header if you set this header
     (used for HTTP Authentication).

   See also headers_sent(), setcookie(), and the section on HTTP
   authentication.
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