problem with headers to prevent cacheing

carl_

Hello everyone, I hope someone helpful could solve this problem.

I'm trying to prevent cacheing of a web page by using the following HTTP headers placed at the top of the script:

<?php
HEADER("Expires: time()");
HEADER("Pragma: no-cache");
HEADER("cache-control: no-cache, no-store, must-revalidate");
?>

But instead, I always receive the following message:

"Warning: Cannot add header information - headers already sent by
(output started at c:\phpweb/test.php:2) in c:\phpweb/test.php on line 3

Warning: Cannot add header information - headers already sent by
(output started at c:\phpweb/test.php:2) in c:\phpweb/test.php on line 4

Warning: Cannot add header information - headers already sent by
(output started at c:\phpweb/test.php:2) in c:\phpweb/test.php on line 5"

I've also tried re-starting Apache, and even re-booting. But the problem remains.

Many thanks.

Anon

you are sending out something to the browser before placing the headers. look to see if you are prining or echoing something before the header calls.

Anon

Be sure that [?php is the first thing to be printed in the file ([ should be the regular tag of course, not sure how this forum handles it).

/Thomas

carl_

Having nothing before the header was the answer to the problem, thank you.

However, I don't understand why should anything before a header statement be treated as header information before the real headers are read. Can anyone enlighten?

kparker

I don't understand why should anything before a header
statement be treated as header information before the real headers are
read.

That's not quite what's happening. Rather, anything in the file before the first php opening tag is treated as content to be sent out. Once the PHP engine sees any such thing (and this would apply to any echo, print, or printf statements, too) it has to close off the header and start the http content.