Warning: A non-numeric value encountered

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...
}

Not exactly the issue you had but the same error for people searching.

This happened to me when I spent too much time on JavaScript.

Coming back to PHP I concatenated two strings with + instead of . and got that error.

You can solve the problem without any new logic by just casting the thing into the number, which prevents the warning and is equivalent to the behavior in PHP 7.0 and below:

$sub_total += ((int)$item['quantity'] * (int)$product['price']);

(The answer from Daniel Schroeder is not equivalent because $sub_total would remain unset if non-numeric values are encountered. For example, if you print out $sub_total, you would get an empty string, which is probably wrong in an invoice. - by casting you make sure that $sub_total is an integer.)