Output buffering in PHP

I was cleaning up the code of a small php application that I wrote long time back. The application was working fine on an old hosting account where it was sitting untouched for almost two years. I was running this application on my local LAMPP installation when I encountered this error:

"Warning: Cannot modify header information - headers already sent by (output started at /opt/lampp/htdocs/sec-app/index.php:35) in /opt/lampp/htdocs/sec-app/index.php on line 86"

Now I was a bit surprised (I don't have much experience in PHP) since this was running fine in that old server and both this and my lampp instance were in PHP5/Apache 2. After searching for sometime I found out that this was due to multiple calls to the php header() function even after the script started outputting html. The remedy was to enable PHP output buffering which I did in the php.ini file. Before going into the configuration, something about output buffering:

Normally, session, cookie or HTTP header data in a PHP script must be sent before any output is generated by the script. If this is not possible in your application, you can enable what PHP calls output buffering, with the output_buffering variable. With output buffering on, PHP stores the output of your script in a special memory buffer and sends it only when explicitly told to do so. This allows you to send special HTTP headers and cookie data even in the middle or at the end of your script;but thsi can have an adverse effect in performance.


output_buffering = Off

You can also pass the output_buffering variable a number indicating the size of the buffer, like:

output_buffering = 1024

I set the value in my php.in as:

output_buffering =4096

That is the default value advised in php.ini. If you want to set this on your production server, you should think about the performance issues. If you don't have access to php.ini on your web server, you can set the same in your .htaccess file as

php_flag output_buffering on

OR

php_value output_buffering 4096

You can add the same setting under a "Directory" directive in apache's httpd.conf file. If even this is not possible or you are not sure about the environment were the script will run, you can enable output buffering in your script itself. Immediately after your php tag opens add the line ob_start(); and immediately before the closing php tag add ob_end_flush(); - or you can enclose the code that causes the error alone within this function duo.

A complete reference of the php.ini variables can be found at: http://www.php.net/manual/en/ini.php