PHP/HTML/headers sent

headers_sent( ) function returns true if your HTTP headers have been sent or false otherwise.

   <source lang="html4strict">

<?

   header("Expires: Sat, 22 Dec 1979 05:30:00 GMT");
   echo "This is some text for output.
";
   if (!headers_sent($filename, $linenum)) {
           header("Location: www.yoursite.ru");
           exit;
   } else {
           echo "Headers already sent in $filename on line $linenum.";
           exit;
   }

?>

 </source>