headers_sent( ) function returns true if your HTTP headers have been sent or false otherwise.
<?
header("Expires: Sat, 22 Dec 1979 05:30:00 GMT");
echo "This is some text for output.<br />";
if (!headers_sent($filename, $linenum)) {
header("Location: www.yoursite.com");
exit;
} else {
echo "Headers already sent in $filename on line $linenum.";
exit;
}
?>