<?php
$myArray = [
 0 => [
  "id" => 1,
  "title" => 'title1'
 ],
 1 => [
  "id" => 2,
  "title" => 'title2'
 ],
 2 => [
  "id" => 3,
  "title" => 'title3'
 ],
 3 => [
  "id" => 4,
  "title" => 'title4'
 ]
];

foreach( $myArray as &$item) {
    $item['funcResult'] = myFunc( $item['id']);
}
unset( $item);

function myFunc($n) { return 10 * $n; }

print_r($myArray);