<label>
						<select name="ilce" id="ilceselect" required>
							<option disabled selected hidden value="0">İl Seçin</option>
						</select>
					</label>

<script type="text/javascript">
		$(function(){
			$("#ilselect").change(function(){
			var ilid = $(this).val();
			console.log(ilid);
               $.ajax({
                   type: 'POST',
                   data: {
                       ilid: ilid
                   },
                   url: 'config/ajax_ilce.php',
                   success: function(data) {  
                       $('#ilceselect').html(data);
                   }
               });    
			})
		})
</script>

<?php 
$ilid = $_POST['ilid'];
?>
<select name="ilce" id="ilceselect" required>
    <option disabled selected hidden>İlçe </option>
		<?php $ilce_select = $db->smartQuery(array('sql' => "SELECT * FROM iller WHERE ilce='$ilid'", 'ret' => 'all'));
		foreach($ilce_select as $select_ilce){ ?>
			<option value="<?=$select_ilce['id'];?>"><?=$select_ilce['isim'];?></option>
		<?php } ?>
</select>