The PHP warning

Warning: Cannot modify header information - headers already sent

is caused, because there’s output (from PHP) already, when there’s a call to e.g. header(), which cannot send a HTTP header then anymore.

If you get this error, it is most likely you recently changed something in a file (e.g. a configuration file in /conf).

The error is often caused by added whitespace (space or line feed) *before* the first <?php or *after* the closing ?>.

You can remove the error by opening the file stated in the notice and remove the whitespace. Because whitespace is often hard to detect, you may want to use a fresh copy of the file and make the changes anew.

Do use a plain text editor like Notepad++ on Windows or textmate on Mac OS for editing B2evo files. Do not use a HTML editor or Word.

Usually the location of the error is given in the error message itself:

 Warning: Cannot modify header information - headers already sent by
 (output started at /path/to/file/with/output:LINE)
 in /path/to/file/with/warning on line X

Take a look at /path/to/file/with/output, especially at/around line LINE.

If there’s no "output started at" info given, you’re kinda out of luck. The only possible solution (apart from ‘’taking a look at the file’s you’ve modified'’) seems to be adding manually echo "foo"; statements into the code to see if the error already gets triggered here (then the /path/to/file/with/warning and "line X" would change accordingly).