Senin için küçük bi kod yazdım.

index.php
<?php ob_start(); session_start(); ?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled Document</title>
</head>

<body>
<?php
		if ( $_POST )
		{
			if ( $_POST['ka'] == "admin" && $_POST['sf'] == "123456" )
			{
				$_SESSION['admin'] = 1;
				header("Location: admin.php");
				exit;
			}
			else
			{
				echo '<font color="red">Kullanıcı adı veya şifre yanlış.</font>';
			}
		}
?>
<form action="index.php" method="post">
	Kullanıcı adı : <input type="text" name="ka" /><br />
    Şifre : <input type="password" name="sf" /><br />
    <input type="submit" value="Giriş" />
</form>
</body>
</html>
admin.php
<?php ob_start(); session_start(); ?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled Document</title>
</head>

<body>
<?php
		if ( $_SESSION['admin'] != 1 )
		{
			echo 'Giriş yapmamışmışsınız. <a href="index.php">Giriş</a>';
		}
		else
		{
			echo 'Hoşgeldin admin...';
		}
?>
</body>
</html>