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.
New parameters: 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.