login prototype

<?php
include "connect.php";
$username= isset($_POST['username'])?$_POST['username']:NULL;
$password= isset($_POST['password'])?$_POST['password']:NULL;
if (!empty($username) && !empty($password))
{
  $username = mysql_real_escape_string(stripslashes($username));
  $password = mysql_real_escape_string(stripslashes($password));
  $sql="SELECT * "
      ."FROM login "
      ."WHERE username='".$username."'"
      ."AND password='".$password."'";
  if($result=mysql_query($sql))
  {
    if(mysql_num_rows($result)==1)
    {
      $r=mysql_fetch_array($result);
      session_start();
      $_SESSION['username']=$r['username'];
      $_SESSION['status']=$r['status'];
    }
    else
    {
      echo "No data founds";
    }
  }
}
{
  echo "<script>alert('isi username dan password anda!');javascript:history.go(-1);</script>";
  exit;
}
?>