How to calculate the discount percentage of the current price and the previous price?

How can I show the percentage in percent %of how much is the value that the product discount is being applied with the current price and the previous price.

Example:

If in the database I have the following records in the tableproducts

id_pro     price     price_old
  1        50.00      100.00

The next message to show on the card would be $50.00 / 50% discount

I perform the query as follows:

Note: The query code PHPis above theHTML

  if (isset($_GET['id'])){
    $id = $_GET['id'];
    $sql = "SELECT * FROM products WHERE url='".$id."'";
    $result = mysqli_query($con, $sql);

    if(mysqli_num_rows($result) > 0){
        while ($row = mysqli_fetch_array($result)) {
            $id = $row['id_pro'];
            $price = $row['price'];
            $price_old = $row['price_old'];
        }
    }
  }

I show the results of the query as follows, within any content HTML.

<?php echo $price; ?>

My idea is to be able to take the value of the two variables $priceand $price_oldthrough them show what percentage of discount is being made.

Note: Always taking into account whether or not there is a previous price registered in the table, some products will not have a previous price registered in that case there would not be a discount percentage in price_oldthe table columnproducts

My idea would be something like this, but I have problems in the approach of the code PHPto show the percentages taking into account the current price and the previous price.

}else{
  echo "Aquí el porcentaje -> 50% de descuento";
}else {
  echo "No existe descuento -> En este caso no se mostrara un texto, quedara un echo vació";
}