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PHP : Function Reference : Network Functions : headers_sent


Checks if or where headers have been sent (PHP 4, PHP 5)
bool headers_sent ( [string &file [, int &line]] )

Example 1603. Examples using headers_sent()


// If no headers are sent, send one
if (!headers_sent()) {

// 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)) {

// You would most likely trigger an error here.
} else {

"Headers already sent in $filename on line $linenum\n" .
"Cannot redirect, for now please click this <a " .
"href=\"\">link</a> instead\n";


Code Examples / Notes » headers_sent


header("Cache-Control: private, must-revalidate, max-age=3600, post-check=3600, pre-check=3600");
////header("Last-Modified: " . gmdate("D, d M Y H:i:s",getlastmod())." GMT");
header("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT");
flush(); // <= (*1)
if (!headers_sent()) {
 header('Content-Type:text/html; charset='._CHARSET);
 header('Expires: Mon, 26 Jul 1997 05:00:00 GMT');
 //header('Last-Modified: '.gmdate('D, d M Y H:i:s').' GMT');
 header('Cache-Control: private, no-cache');
 header('Pragma: no-cache');
headers_sent() does not evaluate it as true, unless the flush()(*1) has been done.
It seems that it does not mean header was sent unless a header output is taken
out to the exterior of PHP.
Apache 2.0.53 (prefork)
PHP 5.0.3 (server module)
... And XOOPS
I had seldom paid attention to flush() on PHP which is not C.
However, it might have been a required thing.
"http ://myhost.mydomain/xoops/modules/test.php?i=1" | less
 % Total    % Received % Xferd  Average Speed          Time             Curr.
                                Dload  Upload Total    Current  Left    Speed
 0     0    0     0    0     0      0      0 --:--:--  0:00:00 --:--:--     0
HTTP/1.1 200 OK
Date: Fri, 22 Apr 2005 05:00:11 GMT
Server: Apache
X-Powered-By: PHP/5.0.3
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: private, must-revalidate, max-age=3600, post-check=3600, pre-check=3600
Pragma: no-cache
Last-Modified: Fri, 22 Apr 2005 05:00:11 GMT
Transfer-Encoding: chunked
Content-Type: text/html
(*)"http :" is "http:" in fact.


To K.Tomono:
Headers are not sent as soon as you call the header() function. They are only sent as soon as some body content (i.e. HTML via echo or escaping from PHP parsing mode) is reached (or, like you did, you send a flush.) Thus after calling header a few times at the beginning they are still unsent and when you call headers_sent() it knows they haven't been sent and reports so. Only when the script ends or you output from content do all the headers so far send.
I think you misunderstood the way they are done and hopefully this should clear a few things up for you.


Re: php at fufachew dot com
That's a nice example how to implement Location header in a correct way (using absoluteURI). 95% of the scripts I have seen just use relativeURI which is wrong. Some browsers, for example lynx, actually notify user about incomplete Location headers. However it might be safer to use $_SERVER['SERVER_NAME'] instead of $_SERVER['HTTP_HOST']. Host header is a HTTP/1.1 feature and you can not count on that if you want to be interoperable with HTTP/1.0 implementations.


RE: antti at haapakangas dot net's post
I've changed the code so $_SERVER['SERVER_NAME'] is used if $_SERVER['HTTP_HOST'] is not set.  $_SERVER['SERVER_NAME'] doesn't meet my needs, but I suppose it's good to fall back on it.  I've also fixed a problem in the meta refresh line - it was missing the "url=" part of the content attribute.
function server_url()
   $proto = "http" .
       ((isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] == "on") ? "s" : "") . "://";
   $server = isset($_SERVER['HTTP_HOST']) ?
   return $proto . $server;
function redirect_rel($relative_url)
   $url = server_url() . dirname($_SERVER['PHP_SELF']) . "/" . $relative_url;
   if (!headers_sent())
       header("Location: $url");
       echo "<meta http-equiv=\"refresh\" content=\"0;url=$url\">\r\n";

trevize shtrudel

Note that in IIS (or at least the version that comes with W2K server), the server seems to do some buffering, so even if you output someting or cause a warning, the value of headers_sent() may be false because the headers haven't been sent yet.
So it's not a safe way to know if warnings have been encountered in your script.


In response to K.Tomono and alexrussell101 at gmail dot com :
headers_sent() will return false, even if you sent something to the ouptut using print() or header() , if output_buffering is different from Off in you php.ini, and the length of what you sent does not exceed the size of output_buffering.
To test it, try this code with  these values in php.ini
1) output_buffering=32
2) output buffering = 4096
   echo "Yo<br />";
   echo "Sent:",headers_sent(),"<br />";
   echo "enough text to feed the buffer until it overflows ;-)<br />";
   echo "Sent:",headers_sent(),"<br />";
then put
3) output buffering = Off
and try this code
   echo "Yo<br />";
   echo "Sent:",headers_sent(),"<br />";
which will this time unconditionnally say that headers were sent.
This is noticed in php.ini comment :
"Output buffering allows you to send header lines (including cookies) even after you send body content, in the price of slowing PHP's output layer a bit."
Note : This is completly independant of implicit_flush tuning.


In responce to: Terry 11-Feb-2005 03:58
if PHP is run as a Module, then it will behave as you describe
However if PHP is run as a CGI then it will be behave like Perl, (which uses CGI unless its mod_perl), as this is a CGI behaviour.


In my case, when I install PHP 5.2.1 in CGI mode under Apache 2.2.3 (on windows 2003 SP2),
function sent_headers() always return false. flash(), ob_end_flash(), so on... no matter.
I suppose, Apache buffering all PHP output until exit()


In case this comes up with anyone else, you might trigger headers to be sent if you have a PHP file with extra space after the closing ?>.  In particular, if you include that file at the top of your main script, it will cause headers to send, followed by the space after the ?> in your included script.  In short, make sure you don't have any space trailing your final ?>.


If you are using output buffering and you use the flush() command ANYWHERE headers_sent() will return true - even if the buffer is seemingly empty.


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.


For programmers used to Perl, note that sending a relative 'Location:' header sends a redirect to the browser in PHP, unlike Perl which will attempt to call relative URLs using an internal subrequest and return that page to the browser without redirecting.  If you want to do the same trick in PHP, use include() or virtual().

jakob b.

function redirect($filename) {
if (!headers_sent())
header('Location: '.$filename);
else {
echo '<script type="text/javascript">';
echo 'window.location.href="'.$filename.'";';
echo '</script>';
echo '<noscript>';
echo '<meta http-equiv="refresh" content="0;url='.$filename.'" />';
echo '</noscript>';

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