Magento Debug HEADERS ALREADY SENT error

Question

I am receiving the following error in my system.log file:

 2011-01-12T14:16:52+00:00 DEBUG (7): HEADERS ALREADY SENT: 
 [0] C:\\xampp\\htdocs\\www.mysite.com         


        

Answer 1

Here's the hard way.

Find the location in the file that's doing the logging

C:\xampp\htdocs\www.mysite.com\app\code\core\Mage\Core\Controller\Response\Http.php 
Mage::log('HEADERS ALREADY SENT: '.mageDebugBacktrace(true, true, true));

Add logging to get a copy of every file included/required so far

Mage::log(print_r(get_included_files(),true));

You can add this logging directly to the core file if you remember to restore the file to it's pre messed with condition, or you can add a temporary copy at

app/code/local/Mage/Core/Controller/Response/Http.php

as long as you remember to remove it when you're done (or just use git).

Check this list of files for the usual white-space suspects, and then check them for any functions that might produce output (echo, print, readfile, there's probably more)