Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by

<?php
ob_start();
session_start();
include "connect.php";
?>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<?
$_SESSION['usr']=$_POST["username"];
$pass=$_POST["pass"];

if (($_SESSION['usr']=="")||($pass=="")) {
  echo "<script>alert('Username หรือ Password ยังไม่ได้กรอก');history.back();</script>";
}

$usr = $_SESSION['usr']; 

$sql = "SELECT * FROM  student, WHERE username = '$usr' and psw = '$pass'";
$result = mysql_query($sql) or die ("Select Error 1");
$num=mysql_num_rows($result);

if ($num>0) 
{
  		$_SESSION['sta'] = "student";
		session_register("username");
		echo "<script> window.location ='studentPage.php'; </script>";
}
else		
{
		$num = 0;	
		$sql = "SELECT * FROM teacher WHERE username = '$usr' and psw = '$pass'";
		$result = mysql_query($sql) or die ("Select Error 2");
		$num=mysql_num_rows($result);
		if($num>0)
		{
			$_SESSION['sta'] = "teacher";
			session_register("username");
			echo "<script> window.location ='teacherPage.php'; </script>";
			
		}
		else
		{		
			$sql = "SELECT * FROM admin WHERE username = '$usr' and psw = '$pass'";
			$result = mysql_query($sql) or die ("Select Error 3");
			$num=mysql_num_rows($result);
			if($num>0)
			{
				$_SESSION['sta'] = "admin"; 
				session_register("username");
				echo "<script> window.location ='adminPage.php'; </script>";
			}
			else
			{
				echo "<script>alert('Username หรือ Password ไม่ถูกต้อง');history.back();</script>";
			}
		}
}

ob_end_flush();
mysql_close();
?>

<?php
session_start();
?>

<?php
include "connect.php"; 
echo "<meta http-equiv='Content-Type' content='text/html; charset=utf-8'> ";
 
ob_start(); 
session_start(); 
$_SESSION['usr']=$_POST['username']; 
$pass=$_POST['pass']; 

if ($_SESSION['usr'] == "" || $pass == "") { 
  echo "<script>alert('Username หรือ Password ยังไม่ได้กรอก');history.back();</script>"; 
} 

$usr = $_SESSION['usr'];  

$sql = "SELECT * FROM  student, WHERE username = '$usr' and psw = '$pass'"; 
$result = mysql_query($sql) or die ("Select Error 1"); 
$num=mysql_num_rows($result); 

if ($num > 0)  
{ 
        $_SESSION['sta'] = "student"; 
        session_register("username"); 
        echo "<script> window.location ='studentPage.php'; </script>"; 
}else{ 
        $num = 0;     
        $sql = "SELECT * FROM teacher WHERE username = '$usr' and psw = '$pass'"; 
        $result = mysql_query($sql) or die ("Select Error 2"); 
        $num=mysql_num_rows($result); 

        if($num > 0) 
        { 
            $_SESSION['sta'] = "teacher"; 
            session_register("username"); 
            echo "<script> window.location ='teacherPage.php'; </script>"; 
             
        }else{         
            $sql = "SELECT * FROM admin WHERE username = '$usr' and psw = '$pass'"; 
            $result = mysql_query($sql) or die ("Select Error 3"); 
            $num=mysql_num_rows($result); 

            if($num > 0) 
            { 
                $_SESSION['sta'] = "admin";  
                session_register("username"); 
                echo "<script> window.location ='adminPage.php'; </script>"; 
            }else{ 
                echo "<script>alert('Username หรือ Password ไม่ถูกต้อง');history.back();</script>"; 
            } 
      } 
} 

ob_end_flush(); 
mysql_close(); 
?>