<form action="?islem=ogrenci" method="post">
Öğrenci İsmini Seçiniz <select size="1" name="kategori">
      <option selected>

    </option>
<?
       include('ayar.php');

      $sql=mysql_query("SELECT ogrenci_adi, ogrenci_sinifi, FROM ogrenciler");

      while($yaz=mysql_fetch_array($sql)){

   ?>

      <option value="<? echo $yaz["ogrenci_sinifi"];?>"><? echo $yaz["ogrenci_adi"];?></option><? }?>
            </select> <input type="submit" value="Sınıfına Bak">
</form>
<? if($_GET["islem"]=='ogrenci'){?>
Öğrenci İsmini Seçiniz <select size="1" name="kategori">
<? 
$sinif=$_POST['kategori'];
include("ayar.php");
$sq=mysql_query("SELECT * from sinif where sinif_id='$sinif'");
$sqa=mysql_query("SELECT * from ogrenciler where ogrenci_sinifi='$sinif'");
$ya=mysql_fetch_array($sq)
$yaa=mysql_fetch_array($sqa)
?>
      <option value="<? echo $yaa["ogrenci_sinifi"]?>"selected>
<? echo $yaa["ogrenci_adi"]?>
    </option>
<?
       include('ayar.php');

      $sql=mysql_query("SELECT ogrenci_adi, ogrenci_sinifi, FROM ogrenciler");

      while($yaz=mysql_fetch_array($sql)){

   ?>

      <option value="<? echo $yaz["ogrenci_sinifi"];?>"><? echo $yaz["ogrenci_adi"];?></option><? }?>
            </select> <? echo $ya["ogrenci_sinifi"]?> <input type="submit" value="Sınıfına Bak">
</form>
<? }?>