A non well formed numeric value encountered

Hi, I'm on to build a ecommerce with laravel 7. But now I'm stuck with this error: A non well formed numeric value encountered. I have searched for a solution online but I can not find any help. My course leader can not answer that either.

In the top of the file I has this lines of code.

@php $featured = DB::table('products')->where('status',1)->orderBy('id','desc')->limit(12)->get();

$trend = DB::table('products')->where('status',1)->where('trend',1)->orderBy('id','desc')->limit(8)->get();

$best = DB::table('products')->where('status',1)->where('best_rated',1)->orderBy('id','desc')->limit(8)->get();

@endphp

                  <div class="product_extras">

                        <div class="product_color">

                            <input type="radio" checked name="product_color" style="background:#b19c83">

                            <input type="radio" name="product_color" style="background:#000000">

                            <input type="radio" name="product_color" style="background:#999999">

                        </div>

                        <button class="product_cart_button">Lägg i kundvagnen</button>

                    </div>

                </div>

                <div class="product_fav"><i class="fas fa-heart"></i></div>

                <ul class="product_marks">


                    @if($row->discount_price == NULL)

                    <li class="product_mark product_discount" style="background: blue;">Ny</li>

                    @else

                    <li class='product_mark product_discount'> 😢 This line is marked as an error

                    @php

                        $amount=$row->selling_price - $row->discount_price;

                        $discount=$amount/$row->selling_price*100;

                    @endphp


                    {{ intval($discount) }}



                    </li>

                    @endif



               </ul>

            </div>

        </div>

I'm a beginner, you could say. Have taken a few courses, but I do not solve this problem.

Best regards JanOve