ช่วยดูตรงนี้ให้ทีครับ เกี่ยวกับ cookie อ่ะครับ...Warning: Cannot modify header information - headers already sent by (out

<?php
	if ( $username=="root" && $password=="1234" )
	{
		print ("welcome<br>");
		print ("click <a href='private.php'>here</a> to private page");
		setcookie ( "u" , "$username" ,time()+3600 );
		setcookie ( "p" , "$password" ,time()+3600 );
	}
	else
	{
		print ("error");
	}
?>

<?php
	if ($u=="root" && $p=="1234")
	{
		print ("welcome to private page<br>");
		print ("<a href='logout.php'>logout</a>");
	}
	else
	{
		print ("Can't open this page<br>");
		print ("Please <a herf='login.html'>login</a>");
	}	
?>

<?php
ob_start();
session_start();
// สองอันนี้ไว้บรรทัดแรกสุดของ page
?>

<?ob_start();?>
<?php

?>

<?php
<?ob_start();?>
<?php
	if ( $username=="test" && $password=="1234" )
	{
		print ("welcome<br>");
		print ("click <a href='private.php'>here</a> to private page");
		
		setcookie ( "u" , "$username" , time()+3600 );
		setcookie ( "p" , "$password" , time()+3600 );
	}
	else
	{
		print ("error");
	}
?>
?>