nur-sery/nur_tests/mapper/base/impl/ReduceMapper.php

<?php
namespace nur\mapper\base\impl;

use nur\mapper\base\Mapper;

class ReduceMapper extends Mapper {
  private static $sum;

  static function sum(): int {
    return self::$sum;
  }

  function setup(): void {
    self::$sum = 0;
  }

  function mapper($item) {
    return self::$sum += $item;
  }
}