php man page for headers_sent

HEADERS_SENT(3) 							 1							   HEADERS_SENT(3)

headers_sent - Checks if or where headers have been sent

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

DESCRIPTION
	Checks if or where headers have been sent.

	You  can't add any more header lines using the header(3) 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.

PARAMETERS
	      o $file
		- If the optional $file and $line parameters are set, headers_sent(3) will put the PHP source file name and line number where out-
		put started in the $file and $line variables.

	      o $line
		- The line number where the output started.

RETURN VALUES
       headers_sent(3) will return FALSE if no HTTP headers have already been sent or TRUE otherwise.

EXAMPLES
       Example #1

	      Examples using headers_sent(3)

	      <?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
" .
			"Cannot redirect, for now please click this <a " .
			"href="http://www.example.com">link</a> instead
";
		  exit;
	      }

	      ?>

NOTES
       Note

	       Headers will only be accessible and output when a SAPI that supports them is in use.

SEE ALSO
       ob_start(3), trigger_error(3), headers_list(3), header(3) for a more detailed discussion of the matters involved. .

PHP Documentation Group 													   HEADERS_SENT(3)