   #PHP Manual HTTP functions headers_list setcookie

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                                 headers_sent

   (PHP 3>= 3.0.8, PHP 4 )
   headers_sent -- Checks if or where headers have been sent

Description

   bool headers_sent ( [string &file [, int &line]])

   headers_sent() will return FALSE if no HTTP headers have already been
   sent or TRUE otherwise. If the optional file and line parameters are
   set, headers_sent() will put the PHP source file name and line number
   where output started in the file and line variables.

   You can't add any more header lines using the header() function once
   the header block has already been sent. Using this function you can at
   least prevent getting HTTP header related error messages. Another
   option is to use Output Buffering.

     Note: The optional file and line parameters where added in PHP
     4.3.0.

   Example 1. Examples using headers_sent()
   <?php
   // If no headers are sent, send one
   if (!headers_sent()) {
       header('Location: http://www.example.com/');
       exit;
   }

   // An example using the optional file and line parameters, as of PHP
   4.3.0
   // Note that $filename and $linenum are passed in for later use.
   // Do not assign them values beforehand.
   if (!headers_sent($filename, $linenum)) {
       header('Location: http://www.example.com/');
       exit;
   // You would most likely trigger an error here.
   } else {
       echo "Headers already sent in $filename on line $linenum\n" .
             "Cannot redirect, for now please click this <a " .
             "href=\"http://www.example.com\">link</a> instead\n";
       exit;
   }
   ?>

   See also ob_start(), trigger_error(), and header() for a more detailed
   discussion of the matters involved.
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