选择查询不工作的PHP

I make an ajax from another page to my php script and it does some computations on the result obtained there and then return those values.

<form id="couponForm">
Apply coupon: <input type="text" name="addCode" placeholder="Apply Coupon">
<input class="button"  style="margin-left:10px;" type="submit" value="Apply">
</form>
<script>
     $('#couponForm').on('submit',function(e){
    e.preventDefault();
    var formData=$(this).serialize();


    var totalPrice=<?php if(isset($total))echo $total;else echo'"'.'"';?>;

    formData=formData+'&totalPrice='+totalPrice;

    $.ajax({
            url:'/cart/couponAjax.php',
            type:'POST',
            dataType:'json',
            data:formData,
            success: function (content) {
              //something
            }

    });
    })
</script>

The above page calls a php script which looks like this

     require 'config.php';// contains db credentials
     $totalPrice=$_POST['totalPrice'];
     $couponName=mysqli_real_escape_string($mysqli,$_POST['addCode']);
     $state=$mysqli->prepare('select * from `coupons` where `name`=?');
     $state->bind_param('s',$couponName);
     $state->execute();
     $result=$state->get_result();

    // $result returns everything as null.

When i check the value obtained in the $result , it shows me something like this

DB Looks something like this

I am stuck in this thing for hours , what am i doing wrong here ?

Update: