How to store checkbox 'checked' status in a session?

I'm writing a website in which there is a galery of photos. Under every photo there is a form with a single checkbox, so i can check it and it should stay checked during the session. The problem is that nothing that I wrote was working and every time I checked the checkbox and hit submit, it keeps unchecking. Here is my form:

<?= $i = 0; ?>
<?php if (count($photos)): ?>
    <?php foreach ($photos as $photo): ?>
    <?= $checkbox['_id'] = $photo['_id']; ?> 
        <div class="gallery">
            <a target="_blank" href="view?id=<?= $photo['_id'] ?>">
                <img src="images/<?=$photo['title']."_min.".$photo['extension']?>" alt="Toster" >
            </a>
            <div class="desc">Tytuł: <?=$photo['title'] ?> </div>
            <div class="desc">Autor: <?=$photo['author'] ?></div>
            <div class="desc"><a href="delete?id=<?= $photo['_id'] ?>">Delete</a></div> 
            <div class="desc">
            <form action="remember_ses" method="post">
            <input type="checkbox" name="checkbox[<?=$i ?>]" value="wybrane" ><?= $photo['_id'] ?>
            <input type="hidden" name="id" value="<?=$checkbox['_id']; ?>">
            <input type="submit" value="Remember" id="submit">
            </form>




            </div>

        </div>
        <?=$i=$i + 1; ?>
    <?php endforeach ?>