Seem to be having problem with Check new Username extension by felix for php,
I keep getting the following error msg

"Warning: Cannot modify header information - headers already sent by (output started at E:\Site\login\adduser.php:2) in E:\Site\login\adduser.php"


$FF_flag="MM_insert";
if (isset($HTTP_POST_VARS[$FF_flag])) {
$FF_dupKeyRedirect="Usererror.php";
$FF_dupKeyUsernameValue = $HTTP_POST_VARS["username"];
$FF_dupKeySQL = "SELECT username FROM users WHERE username='" . $FF_dupKeyUsernameValue . "'";
mysql_select_db($database_password_entry, $password_entry);
$FF_rsKey=mysql_query($FF_dupKeySQL, $password_entry) or die(mysql_error());
if(mysql_num_rows($FF_rsKey) > 0) {
// the username was found - can not add the requested username
$FF_qsChar = "?";
if (strpos($FF_dupKeyRedirect, "?") $FF_qsChar = "&";
$FF_dupKeyRedirect = $FF_dupKeyRedirect . $FF_qsChar . "requsername=" . $FF_dupKeyUsernameValue;
header ("Location: $FF_dupKeyRedirect";
exit;
}


Seems to be complaining about the line above exit.

any help is appreciated and it goes without saying that my php knowledge is limited so go easy please