Bypassing af min tjek kode

Jeg er igang med at lave et loginsystem i php + mysql ved brug af nedenstående check script.

-------------------------------------------
<?php
ob_start();
$host="localhost"; // Host name
$username=""; // Mysql username
$password=""; // Mysql password
$db_name=""; // Database name
$tbl_name="members"; // Table name

// Connect to server and select databse.
mysql_connect("$host", "$username", "$password")or die("cannot connect");
mysql_select_db("$db_name")or die("cannot select DB");

// Define $myusername and $mypassword
$myusername=$_POST['myusername'];
$mypassword=$_POST['mypassword'];

// Encrypt password
$encrypted_mypassword=md5($mypassword);

// To protect MySQL injection (more detail about MySQL injection)
$myusername = stripslashes($myusername);
$mypassword = stripslashes($mypassword);
$myusername = mysql_real_escape_string($myusername);
$mypassword = mysql_real_escape_string($mypassword);

$sql="SELECT * FROM $tbl_name WHERE username='$myusername' and password='$encrypted_mypassword'";
$result=mysql_query($sql);

// Mysql_num_row is counting table row
$count=mysql_num_rows($result);
// If result matched $myusername and $mypassword, table row must be 1 row

if($count==1){
// Register $myusername, $mypassword and redirect to file "login_success.php"
session_start();
$_SESSION["myusername"] = $myusername;
$_SESSION["mypassword"] = $mypassword;
header("location:login_success.php");
}
exit();

else {
header("location:login_error.php");
}
exit();

ob_end_flush();
?>
-------------------------------------------

Jeg har af en eller anden grund adgang til login.success.php ved bare at skrive localhost/login.success.php og bypass'er nærmest tjekket, som vist herunder:

<?
session_start();
if(!isset($_SESSION['myusername'])){
header("location:index.php");
exit();
}
?>