$il_id=$_POST['il_id'];        
$rv='';

$ilcelersor=$db->prepare("SELECT * FROM ilceler where il_id='.$il_id.' order by id ASC");
$ilcelersor->array();
while($ilcelercek=$ilcelersor->fetch(PDO::FETCH_ASSOC)){

 $rv.='<option value="'.$ilcelercek['id'].'">'.$ilcelercek['adi'].'</option>'; }

 echo $rv;

$il_id=$_POST['il_id'];        
$rv='';

$ilcelersor=$db->prepare("SELECT * FROM ilceler where il_id=? order by id ASC");
$ilcelersor->execute([$ilid]);
while($ilcelercek=$ilcelersor->fetch(PDO::FETCH_ASSOC)){

 $rv.='<option value="'.$ilcelercek['id'].'">'.$ilcelercek['adi'].'</option>'; }

 echo $rv;