$_sum = 0;
foreach ($sum as $key => $value) {
  $_sum += $key * $value;
}
echo $_sum;

. I don't think there's anything wrong with the Ls answer. It's obviously a summary value, so why cycle through echo ?

I don't quite understand how your data structure is planned here. This should be a two-dimensional array structure.
in addition, you used number_format above. This output is a string. It is recommended that you use it before rendering the page after all calculations, because the computer's native floating-point numbers will have accuracy errors in calculation (you can search "IEEE754 accuracy error" by yourself), and the type is also wrong.

I don't understand your question, but I wonder if it has something to do with the reference value of foreach