Parent ID Tag

I searched and found “Snippet 2” on this page:
http://codex.wordpress.org/Conditional_Tags
I’ve added that function to my functions.php, but when I add several variations of

<?php is_subpage(); echo $parentID; ?>

to my page.php, nothing is returned. I’m sure I’m just not using the correct syntax. I know this is a child page and I’m trying to return the parent page ID. I can’t find any more info on this unless I’ve just blatantly overlooked it. Thanks.