State based sumation to callback based sumation example.

array.php

<?php
require 'context.php';

$odd = array_flip(array_filter(array_flip($pairs),
   function($key) { return $key % 2 == 0; }
));
$even = array_diff($pairs, $odd);

$sumOdd = array_reduce($odd, Summation::$fSum);
$sumEven = array_reduce($even, ['Summation', 'rSum']);
$sumAll = array_reduce($pairs,  [new Summation(), 'rightSum']);
var_dump($sumOdd, $sumEven, $sumAll);

$withVat = array_map(
  function ($price) { return vatGeneral($price, 21); },
  $pairs);
$sumWithVat = array_reduce($withVat, 'Summation::rSum');
var_dump($withVat, $sumWithVat);

context.php

<?php
class Summation { // array_sum alternative
    static $fSum = null;

    static function rSum($aggregated, $current) {
      return $aggregated + $current;
    }

    function rightSum($aggregated, $current) {
      return self::rSum($aggregated, $current);
    }
}

Summation::$fSum = function($aggregated, $current) {
   return Summation::rSum($aggregated, $current);
};

function vatGeneral($price, $vat) {
  return $price * (1+$vat/100);
}

$pairs = (array) json_decode('{"1":10,"2":5,"3":8,"4":1}');

state.php

<?php
require 'context.php';

$sumOdd = 0;  $sumEven = 0;  $sumAll = 0;
$sumWithVat = 0;
$withVat = array();

foreach ($pairs as $name => $price) {
    if ($price % 2 == 0) {
        $sumOdd += $price;
    } else {
        $sumEven += $price;
    }

    $sumAll += $price;
    $priceWithVat = vatGeneral($price, 21);
    $sumWithVat += $priceWithVat;
    $withVat[$name] = $priceWithVat;
}

var_dump($sumOdd, $sumEven, $sumAll);
var_dump($withVat, $sumWithVat);