Checkbox status saved in 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 kept 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 ?>
   <?php endif ?>

Photos is an array where I store my images.