php de resim üzerinde güvenlik kodu

<?
  set_magic_quotes_runtime(0);
  if (!session_is_registered("kod")) {
  $kod = mt_rand(9999,9999999); //9999 ve 9999999 sayilari arasinda rastgele bir kod oluşturur
  session_register("kod");
  }
  $image = imagecreate(115, 20);
  $white = imagecolorallocate($image, 200, 0, 200);
  $gray = imagecolorallocate($image, 192, 192, 192);
  $black = imagecolorallocate($image, 0, 0, 0);
  $w=imageline($image, 140, 10, 0, 10, $gray);
  $q=imagestring($image, 4, 28, 2, $_SESSION['kod'], $black);
  imagepng($image,$q,$w);
  imagedestroy($image);
  ?>

<?
session_start();
if (!$_POST) {
include "resim.php";
?>
<br><br><br><br>
<table border=0 align=center>
<form action="" method="POST">
<tr>
  <td>Kullanıcı adı:</td>
  <td><input type="text" name="adi" size="20"></td>
</tr>
<tr>
  <td>Şifre:</td>
  <td><input type="password" name="sifre" size="20"></td>
</tr>
<tr>
  <td>Güvenlik kodu:</td>
  <td>
  <img src="1" border="0" />
  </td>
</tr>
<tr>
  <td> </td>
  <td><input type="text" name="gkod" size="10"></td>
</tr>
<tr>
  <td> </td>
  <td><input type="submit" value="giriş" class=news></td>
</tr>
</form>
</table>
<?
} else {
if (($_POST['adi'] == "deneme" AND $_POST['sifre'] == "deneme") AND ($_POST['gkod'] == $_SESSION['kod'])) {
session_unregister("kod");
session_register("adi");
header("Location:yonetim.php");
} else {
header("Location:index.php");
}
}
?>

<?
session_start();
if (session_is_registered("adi")) {
echo $_SESSION['adi'];
echo "<br><a href='cikis.php'>Çıkış</a>";
} else {
header("Location:index.php");
}
?>

<?
session_start();
session_destroy();
header("Location:index.php");
?>