php_8/generators/combine_from.php

<?php
function square($value)
{
    yield $value * $value;
}

function even_square($arr) {
    foreach($arr as $value) {
        if ($value % 2 == 0) yield from square($value);
    }
}

$arr = [1, 2, 3, 4, 5, 6];
foreach(even_square($arr) as $val) echo "$val ";