<?php
include 'dbconnect.php';
$cat = $_POST['cat'];
$squery = "select sub_category from category where category_name='$cat'";
$result = mysqli_query($conn,$squery);
$fn ='';
$sq = mysqli_fetch_array($result);
$d = $sq['sub_category'];
$data= explode(',',$d);
for($i=0; $i < count($data);$i++)
{
     $fn.= '<option value="'.$data[$i].'">'.$data[$i].'</option>';
}

 echo $fn;
  ?>