This is the code that I used to verify locally my answer for a concret stackoverflow question

YieldTraining1.php

<?php declare(strict_types=1);
# https://stackoverflow.com/a/58767497/3454593

// Usage example:
$postWithImage = new PostWithImage(
    $chaptersUrlApi = 'https://httpbin.org/get',
    $imagesUrlApi = 'https://httpbin.org/get?{$key}={$value}'
);

foreach ($postWithImage->generate(20) as $post) {
    var_dump($post);
}

// The code:
final class PostWithImage
{
    /** @var string */
    private $chaptersUrlApi;
    /** @var string */
    private $imagesUrlApi;

    public function __construct(string $chaptersUrlApi, string $imagesUrlApi)
    {
        $this->chaptersUrlApi = $chaptersUrlApi;
        $this->imagesUrlApi = $imagesUrlApi;
    }

    public function generate(int $total): Generator
    {
        $headers = $this->getHeaders();
        foreach ($headers as $key => $value) {
            $images = $this->getImagesPerChapter($key, $value);

            foreach ($images as $image) {
                yield $this->makePostWithImage($image->current()['origin']);
            }
        }
    }

    private function getHeaders(): array
    {
        return $this->makeRequest($this->chaptersUrlApi)->current()['headers'];
    }

    private function getImagesPerChapter(string $key, string $value): Generator
    {
        yield $this->makeRequest(sprintf($this->imagesUrlApi, $key, $value));
    }

    private function makeRequest(string $url): Generator
    {
        $curl = curl_init();
        curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, false);
        curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
        curl_setopt($curl, CURLOPT_FOLLOWLOCATION, true);
        curl_setopt($curl, CURLOPT_URL, $url);
        $result = curl_exec($curl);
        curl_close($curl);

        yield json_decode($result, true);
    }

    private function makePostWithImage(string $image): array
    {
        return [
            'post_content' => sprintf('<img src="%s">', $image),
            'post_title' => 'Chapeter 2 !!',
            'post_status' => 'publish',
        ];
    }
}