This message means that your PHP script is sending data to the web browser before the headers and is actually fairly common. To resolve this issue you simply will need to edit the .htaccess file in your public_html folder on your account and add this line:
php_flag output_buffering on
Output buffering causes the web server to process all PHP before sending anything to the browser which allows your script to modify the headers at any time during the execution of the script, eliminating this error message.
