ayar.php
<?
!defined('_ADMIN') and die('Izin yok');
$sifre=12345;
?>
------------------------------------------------------
giris.php
<?
define('_ADMIN', TRUE);
include 'ayar.php';
session_start();
if($_SESSION['sifre']==$sifre){
header('location: panel.php');
exit();
}
if($_POST['a']==1 and $sifre==$_POST['sifre']){
$_SESSION['sifre']=$_POST['sifre'];
header('location: panel.php');
exit();
}
?>
<form action="giris.php" method="post">
<input type="hidden" name="a" value="1">
<input type="text" name="sifre"><br>
<input type="submit" value="Giris">
</form>
------------------------------------------------------
panel.php
<?
define('_ADMIN', TRUE);
include 'ayar.php';
session_start();
if($_SESSION['sifre']==$sifre){
header('location: panel.php');
exit();
}
?>
.....Panel kodları........
------------------------------------------------------

Denemedim ama çalışması lazım