<?php
$rows = $this->getPosts();

foreach ($rows as $row) {
    $row->likes = $this->getLikes($row->id_post);
}
// now you have everything in one object
?>



<?php foreach ($rows as $row): ?>
<div class="POST">
<?php for($i=0; $i<=3; $i++): ?>
<?= $row->likes[$i]->firstname; ?>
// this will loop 3 user likes
<?php endfor; ?>
</div>
<?php endforeach; ?>