You absolutely can, I use this approach a lot w/ both JavaScript and PHP.
Field definition:
<input type="hidden" name="foo" value="<?php echo $var;?>" />
Access w/ PHP:
$_GET['foo'] or $_POST['foo']
Also: Don't forget to sanitize your inputs if they are going into a database. Feel free to use my routine: https://github.com/niczak/PHP-Sanitize-Post/blob/master/sanitize.php
Cheers!