It seems that in PHP 7.1, a Warning will be emitted if a non-numeric value is encountered. See this link.
Here is the relevant portion that pertains to the Warning notice you are getting:
New E_WARNING and E_NOTICE errors have been introduced when invalid strings are coerced using operators expecting numbers or their assignment equivalents. An E_NOTICE is emitted when the string begins with a numeric value but contains trailing non-numeric characters, and an E_WARNING is emitted when the string does not contain a numeric value.
I’m guessing either $item[‘quantity’] or $product[‘price’] does not contain a numeric value, so make sure that they do before trying to multiply them. Maybe use some sort of conditional before calculating the $sub_total, like so:
<?php
if (is_numeric($item['quantity']) && is_numeric($product['price'])) {
$sub_total += ($item['quantity'] * $product['price']);
} else {
// do some error handling...
}