<?php
    $query = "select * from table_a";
    foreach($query as $row):
?>
    <ul>
        <li><?= $row->id_a?></li>
            <?php $queyr = "select * from table_b where id_a = $row->id_a" ?>
            <?php foreach($query as $row):?>
                <ul>
                    <li><?= $row->id_b ?></li>
                </ul>
            <?php endforeach ?>
    </ul>
<?php endforeach ?>