Possible Duplicate:
Headers already sent
Headers already sent by PHP
I have a error on session How to fix this
`Cannot modify header information - headers already sent by (output started at /home/xxxx/public_html/app/libraries/config_data.php:50)
Filename: libraries/Session.php
Line Number: 671`
If you let us see your code, we'll be able to help better.
First you have to understand what headers the parser is talking about. If you send any HTML, including white space or any text, then headers are sent. If you issue a PHP header() call, then headers are sent. If you try to start a session, headers are sent.
Usually the error you mentioned is caused by whitespace or an echo being sent before trying to start a session or issue a header() call.
There's a really good answer at the link provided by @Juhana
output started at /home/xxxx/public_html/app/libraries/config_data.php:50
Something on this line is causing the headers to be set, could you post this code please?