headers_sent
Description
bool headers_sent ( [string &file [, int &line]])headers_sent() will return FALSE if no HTTP headers have already been sent or TRUE otherwise. If the optional file and line parameters are set, headers_sent() will put the php source file name and line number where output started in the file and line variables.
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 HTTP header related error messages. Another option is to use Output Buffering.
Note: The optional file and line parameters where added in PHP 4.3.0.
See also ob_start(), trigger_error(), and header() for a more detailed discussion of the matters involved.