Warning: Cannot modify header information - headers already sent by
<?php   session_start();
    include_once("include/Connection.php");

    $userid=$_POST['userid'];
    $password=$_POST['password'];

    $loginSql = "SELECT * FROM admin2 WHERE  admin_name='$userid' and admin_psw='$password'";
    $loginQuery=mysql_query($loginSql);
    $num_row = mysql_num_rows($loginQuery);
    $login = mysql_fetch_array($loginQuery);

    $user = $login['admin_name'];
    $password = $login['admin_psw'];

    echo $user;
    echo    $password;

    if($num_row >0)
    {
        if($user == $userid)
        {
            $_SESSION['id_gosi'] = $login['id'];
            $_SESSION['userid_gosi'] = $login['admin_name'];
            header("Location: CustomerMaster.php");
            }
      else{
            header("Location: index.php?msg=2");
            exit();}}
  else{
      header("Location: index.php?msg=1");
    }?>

/* echo $user;
   echo $password;*/

if($num_row >0)
{
 if($user == $userid)
 {
  ....
  header("Location: CustomerMaster.php");
 }
 else
 {
  header("Location: index.php?msg=2");
  exit();
 }
}
else
   header("Location: index.php?msg=1");
?>