This function can be used to add the values in one array to another:
<?php
echo '<pre>';
// example code
$a = [ 1, 2, 3, 4, 5, 6];
$c = [10,20,30,40,50,60];
array_walk($a,
function(&$item, $key, $y) { $item += $y[$key];},
$c
);
print_r($a);
// result
Array
(
[0] => 11
[1] => 22
[2] => 33
[3] => 44
[4] => 55
[5] => 66
)