PHP calculating two arrays and then add their sum - php

Question

I want to calculate an array to another array and then add both like this :
Array1(4,8,7,12);
Array2(3,6);
the result is like this :
3x4+3x8+3x7+3x12+6x4+6x8+6x7+6x12 = 279
I tried with this code and since I'm still new with php I still didn't make any tips I'll be glad thanks in advance
<?php
tab1(4,8,7,12);
tab2(3,6);
$s=0;
for($i=0; $i<$tab1[3]; $i++){
for($j=0; $j<$tab2[1]; $j++){
$s = $s + tab[i] * tab[j];
}
echo "$s";
}
?>

Answer 1

This is actually a piece of cake. I am not sure what tab1 means but I assume you are trying to use this function to get an array of desired(input) numbers.
Using a foreach loop to loop over the two arrays, I came up with the code below:
<?php
$a = [4,8,7,12];
$b = [3,6];
$sum = 0;
foreach ($b as $valueInB) {
foreach ($a as $valueInA) {
$sum += $valueInA * $valueInB;
}
}
echo $sum;
?>
which results in:
279
if you are insisting on using a for loop you can run the code below to get the same result:
<?php
$a = [4,8,7,12];
$b = [3,6];
$sum = 0;
$bLength = count($b);
$aLength = count($a);
for ($i=0; $i < $bLength ; $i++) {
$valueInB = $b[$i];
for ($j=0; $j < $aLength ; $j++) {
$valueInA = $a[$j];
$sum += $valueInA * $valueInB;
}
}
echo $sum;
?>
Please note that you should use the count outside of the loop because looping over the array and calling count each time is not an efficient way. (Thanks to Will B for the comment)

Answer 2

For a more simplistic approach without iteration you can use array_sum on the two arrays.
Example: https://3v4l.org/3gtgE
$a = [4,8,7,12];
$b = [3,6];
$c = array_sum($a) * array_sum($b);
var_dump($c);
Result
int(279)
This works because of the order of operations of:
(3*4) + (3*8) + (3*7) + (3*12) + (6*4) + (6*8) + (6*7) + (6*12) = 279
Equates to the same as the sum of a multiplied by the sum of b:
(4+8+7+12) * (3+6) = 279