Untitled

Page Redirection problem in php
Warning: Cannot modify header information -
headers already sent by (output started at C:wampwwwstudentlogin.php:18)
in C:wampwwwstudentlogin.php on line 19

<?php
echo 'test';
header('location:index.php');
exit;
?>

<html>
  <head> .....
<?php
echo 'test';
header('location:index.php');
exit;
?>

<?php
echo 'test';
ob_start();
flush();
header('location:index.php');
exit;
?>

ob_start();