The message "Cannot modify header information - headers already sent"
is a general PHP warning. It means that when PHP tries to generate
some output it detects that something was already sent. This is
usually an error message that you could view by looking at the source
code of the generated HTML page.
It can also be a special marker indicating a Unicode file that Notepad
inserts in the file. Search the list archives for some information on
this:
http://lists.wikimedia.org/pipermail/mediawiki-l/