sayfa şifreleme formunda hata?!..

<?php
session_start();
if($_post["username"] == 'admin' && $_post["pass"] == 'semih') {
      $_SESSION["oturumvar"]=1;  
}
     else{
     echo "uza";
     }
      session_register();   
     ?>

<?php
    session_start();
 if($_SESSION["oturumvar"]==1) {
    header("Location: liste.php");           
}   else{
  header("Location: siktir.php");           
}
?>

<form name="form1" method="post" action="giris.php">
  <label>
   kullanıcıadı: 
  <input name="username" type="text" id="username">
  </label>
  <p>şifre: 
    <label>
     <input name="pass" type="password" id="pass">
    </label>
  </p>
  <p>
    <label>baglan
    <input type="submit" name="Submit" value="baglan">
    </label>
  </p>
</form>

<?php
    session_start();
 if($_SESSION["oturumvar"]==1) {
    header("Location: liste.php");     
}   else{
  header("Location: yanlisgirdin.php");           
}

?>

<?php
    session_start();
 if($_SESSION["oturumvar"]==1) {
    header("Location: liste.php");     
}   else{
  header("Location: yanlisgirdin.php");           
}
?>

echo $_SESSION["oturumvar"]

print_r($_SESSION);

if($_SESSION["oturumvar"]==1) {
    header("Location: liste.php");           
}   else{
  header("Location: s*kt*r.php");           
}

<?php


$ourLogin = "kullanıcıadı";
$ourPassword = "şifre";

session_start();

if ($_SESSION['login']!=$ourLogin && $_SESSION['password']!=$ourPassword) {


if ($_POST['login']==$ourLogin && $_POST['password']==$ourPassword) {


$_SESSION['login'] = $_POST['login'];
$_SESSION['password'] = $_POST['password'];
header("Location: index.php");


}
else {


echo "
<form action=index.php method=post>


Login:<br>
<input type=text name=login value=''><br>

Password:<br>
<input type=password name=password value=''><br>

<input type=submit value=' - ok - '>


</form>
";

exit;

}

} 

?>