Bir de bu şekilde deneyin.
<?php 
session_start(); 
include("include/sayfa/header.php"); 

echo "<br>"; 
$user = mysql_query("SELECT * FROM ayarlar WHERE id='6'"); 
$kayit = mysql_fetch_array($user); 
$username = $kayit["ayar"];

echo $kayit['ayar']; 
echo "<br>"; 

$pass = mysql_query("SELECT * FROM ayarlar WHERE id='7'"); 
$kayit = mysql_fetch_array($pass); 
$password = $kayit["ayar"];

echo $kayit['ayar']; 
echo "<br>"; 
echo $_POST["user"]; 
echo "<br>"; 
echo $_POST["pass"]; 
echo "<br>"; 

session_start(); 


if($_POST["user"] == $username AND $_POST["pass"] == $password) 
    {  
        $_SESSION["login"] = "true";  
        $_SESSION["user"] = $username;  
        $_SESSION["pass"] = $password;  
    } 
    else 
        { 
            echo "Kullanycı Adı veya Şifre Yanlyş.";  
        } 
?>