$mass = ['one', 'two', 'three'];
if (isset($_POST['signature'])) {
echo $_POST['signatory_id'];
}
<?php foreach ($mass as $m): ?>
<input type="hidden" name="signatory_id" value="<?=$m?>">
<button name="signature" type="submit" class="btn btn-danger"><i class="mdi mdi-pen" style="font-size: 16px;"></i> Подписать</button>
<?php endforeach; ?>