Ошибка - Warning: Cannot modify header information - headers already sent

<? include ("lock.php"); ?>
 
       <?php  
$result10 = mysql_query ("SELECT COUNT(*) FROM data",$db);
$sum = mysql_fetch_array($result10);
 
$result11 = mysql_query ("SELECT COUNT(*) FROM comments",$db);
$sum2 = mysql_fetch_array($result11);
 
function online () {
$ip=getenv("HTTP_X_FORWARDED_FOR");
if (empty($ip) || $ip=='unknown') { $ip=getenv("REMOTE_ADDR"); }
# уд. старые сессии
mysql_query ("DELETE FROM online WHERE UNIX_TIMESTAMP() - UNIX_TIMESTAMP(time) > 300") or die ("Can't delete old sess");
 
# проверка на присутстаие или занесение нового пользователя
$select = mysql_query ("SELECT ip FROM online WHERE ip='$ip'") or die ("Can't select duble");
$tmp = mysql_fetch_row ($select);
if ($ip == $tmp[0]) {
mysql_query ("UPDATE online SET time=NOW() WHERE ip='$ip'") or die ("Can't update");
} else {
mysql_query ("INSERT INTO online (ip,time) VALUES ('$ip',NOW())") or die ("Can't insert");
}
# считывание результатов
$select = mysql_query ("SELECT COUNT(*) FROM online") or die ("Can't select result");
$tmp = mysql_fetch_row ($select);
$result = $tmp[0];
return $result;
} 
 ?>
 <? require_once ("tcpdf/tcpdf.php"); 
 
$tcpdf = new TCPDF("P", 'mm', 'A4', true, 'UTF-8')?>
 <td><? ?></td>
 <?$tcpdf->AddPage();
 $tcpdf->SetTextColor(0, 0, 0);
 $text = "<p class='comments'>Заметок в базе: $sum[0]<br>";
 $tcpdf->Write(3, $text, '', false, 'L');
 $tcpdf->Output('test.pdf');
      ?>