<form method="post">

          <select id="cat_sub_cat">

          <?php
          $data=mysqli_query($con,"SELECT * FROM category");
          while($row=mysqli_fetch_array($data))
          {
              echo "<option value=".$row['cid'].">".$row['category']."</option>";
          }

          ?>

          </select><br><br>

          <h3>Add Sub Categories</h3><br>

          <input type="text" name="sub_cat"><br><br>
          <input type="submit" name="submit" value="Insert" id="sub_cat_btn">
 </form>

$(document).on('click','#sub_cat_btn', function(event){
    event.preventDefault();

    $.ajax({
        url:"sub_cat_add_back.php",
        method:"post",
        data:$('form').serialize(),
        dataType:"html",
        success:function(strMsg){
            $("#cat_sub_msg").html(strMsg);

            }

        })

<?php

include "../includes/config.php";


$name=$_POST['sub_cat'];
$cid=$_POST['cat_sub_cat'];


$data=mysqli_query($con,"INSERT INTO subcategory (name,cid) VALUES ('$name','$cid')");
if($data=="true")
{
    echo "Successfully Inserted";
}
else
{
    echo "Error";
}


?>