birde şunu denermisin,

<?php
if (!empty($_POST[kullaniciadi]) and !empty($_POST[sifre])) {
@mysql_connect ("localhost", "baksakne_kurs", "kurs") or die ("MySQL'e baglanilamadi"); 
@mysql_select_db ("baksakne_kurs") or die ("Veritabanina baglanilamadi"); 

$sql = "select * from uyeler where kullaniciadi='".$_POST[kullaniciadi]."' and sifre='".$_POST[sifre]."' ";
$kontrol = mysql_query($sql);
$kayitsayisi = mysql_num_rows($kontrol);

if ($kayitsayisi == "0") {
header ("Location: uyegirisi.php?hata=yes");
} else { 
$kontrol_ok = mysql_fetch_array($kontrol);
setcookie ("kullanici", "$kontrol_ok[kullaniciadi]");
header ("Location: index.php");
}

}
else {
header ("Location: uyegirisi.php?hata=yes");
}

?>