As the message explains: Do not divide through zero. Your Price is zero.
Option 1: Write a Function
$array = [
'foo' => 'bar',
'donald' => 'duck',
'saving' => GetDiscount($product_info['price'], $product_info['special'])
]
//Get the Discount. If the Price is zero, Discount is 100 %
function GetDiscount($price, $special) {
return $price === 0 ? 100 : round((($price - $special)/$price)*100, 0);
}
Option 2: One Line
$array = [
'foo' => 'bar',
'donald' => 'duck',
'saving' => $product_info['price'] === 0 ? 100 : round((($product_info['price'] - $product_info['special'])/$product_info['price'])*100, 0)
]