Get parent page ID if exists

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<?php

function is_subpage() {
    global $post;
    if ( is_page() && $post->post_parent ) {
        return $post->post_parent;
    } else {
        return false;
    }
}

?>

Instructions:

This is a great little snippet that will let you return the ID of the parent page if the page has a parent.

Related links:

http://wpsnipp.com/index.php/functions-php/return-id-if-parent-page-exists/