This function returns TRUE if the HTTP headers have already been sent, FALSE otherwise. You can't add any more header lines using the header() function once the header block has already been sent. Using this function you can at least prevent getting the "Duplicate headers ..." error messages.
The function will return the php source file and line number where output started in the optional file and line parameters if these are given.
Note: file and line where added for PHP 4.3.0.
See also header() for a more detailed discussion of the matters involved.