lazy_map.php

<?php

function lazy_map(callable $f, Iterator $list): Iterator {
   foreach ($list as $item)
      yield $f($item);
}

function lazy_filter(callable $f, Iterator $list): Iterator {
   foreach ($list as $item)
      if ($f($item))
         yield $item;
}

function tiny_gen(): Iterator {
   foreach (range(1, 10) as $num)
      yield $num;
}

$addOne = function($item) { return $item + 1; };
$isOdd = function($item) { return $item % 2; };

// Using lazy functions
$odds = lazy_filter($isOdd, lazy_map($addOne, tiny_gen()));

// 3
// 5
// 7
// 9
// 11
foreach ($odds as $odd) {
   echo "$odd\n";
}