coroutine.php

<?php

class DataTransformer
{
    public function transform(): \Generator
    {
        while ($request = yield) {
            yield $request . ' - appended';
        }
    }
}

class DataRequester
{
    /** @var DataTransformer */
    private $dataTransformer;

    public function __construct(DataTransformer $dataTransformer)
    {
        $this->dataTransformer = $dataTransformer;
    }

    public function execute(iterable $batches): iterable
    {
        $coroutine = $this->dataTransformer->transform();
        
        foreach ($batches as $batch) {
            while ($result = $coroutine->send($batch)) {
                yield $result . "\n";
            }
        }
    }
}

$requester = new DataRequester(
    new DataTransformer()
);

$it = new \AppendIterator();
$it->append(new \ArrayIterator(['a','b','c']));
$it->append(new \ArrayIterator(['d','e','f']));
$it->append(new \ArrayIterator(['g','h','i']));

foreach ($requester->execute($it) as $value) {
    echo $value . PHP_EOL;
}


/*

Results in:

a - appended
b - appended
c - appended
d - appended
e - appended
f - appended
g - appended
h - appended
i - appended
*/