php session hatası acil yardım

if(isset($_GET['o_ok'])) 
  {	 
  	$o_tc=$_POST['o_tc']; 
  	$o_no=$_POST['o_no']; 
  	$sorgu=mysql_query("select * from ogrenci where tc='$o_tc' && okul_no='$o_no'"); 
  	$kactane=mysql_num_rows($sorgu); 
  	 
  	if($kactane==1) 
  	{ 
  	 
  		$_SESSION['ogrenci']=$o_no; 
  		 
  		?><meta http-equiv="refresh" content="0;?sayfa=ogrenci"/><? 
  	} 
  	else 
  	{?> 
  		<meta http-equiv="refresh" content="0;?sayfa=o_giris&hata"/> 
           
  	<?	 
  	}	 
  } 
   
  else if(isset($_GET['v_ok'])) 
  { 
  	 
  	$v_tc=$_POST['v_tc']; 
  	$v_o_no=$_POST['v_o_no']; 
  	$denemee=mysql_query("select id from ogrenci where okul_no='$v_o_no'"); 
  	echo $denemee; 
  	$sorgu=mysql_query("select * from veli where tc='$v_tc'  && ogrenci_id='$denemee'"); 
  	$kactane=mysql_num_rows($sorgu); 
  		//echo "$kactane"; 
  	if($kactane==1) 
  	{ 
  		$_SESSION['veli']=$v_tc; 
  		 
  		?><meta http-equiv="refresh" content="5;?sayfa=veli"/><? 
  	} 
  	else 
  	{?> 
  		<meta http-equiv="refresh" content="5;?sayfa=v_giris&hata"/> 
  	<? }	 
  }

$denemee=mysql_query("select * from ogrenci where okul_no='$v_o_no'"); 
  	$x=mysql_result($denemee,0,'id');