tweetlist.php

<?php
final class TweetCollection implements \IteratorAggregate, \Countable
{
    public function __construct(array $attributes = [])
    {

    }

    /**
     * @param Tweet $tweet
     */
    public function add(Tweet $tweet): void
    {
        $this->attributes[] = $tweet;
    }

    /**
     * @return int
     */
    public function count(): int
    {
        return count($this->attributes);
    }

    /**
     * @return Tweet[]|ArrayIterator
     */
    public function getIterator()
    {
        return new \ArrayIterator($this->attributes);
    }
}

class Tweet implements \JsonSerializable
{
    private $text = null;
    private $media_ids = null;
    
    public function __construct($text='', $media_ids=[])
    {
        $this->text = $text;
        $this->media_ids = $media_ids;
    }

    public function jsonSerialize() {
        $result = [];
        $result['text'] = $this->text;

        // 画像あれば入れる
        if (count($this->media_ids) > 0) {
            $result['media_ids'] = $this->media_ids;
        }

        return $result;
    }
}

$twlist = new TweetCollection();
$twlist->add(new Tweet('aaaa', ['111', '222']));
$twlist->add(new Tweet('bb', ['3', '5']));

foreach($twlist as $tw){
    var_dump(json_encode($tw));
}