Get the id of the parent page on the root level

Example:

• Root parent page (we need to get the id of this page)
  • Subpage
    • Sub-subpage
      • Sub-sub-subpage (we are here)

Recursive function for getting parent page id on root level in functions.php. Works with any nesting depth:

<?php

function get_root_parent_id( $page_id ) {

	global $wpdb;

	$parent = $wpdb->get_var( "SELECT post_parent FROM $wpdb->posts WHERE post_type='page' AND post_status='publish' AND ID = '$page_id'" );

	if( $parent == 0 ) {

		return $page_id;

	} else {

		return get_root_parent_id( $parent );

	}

}

?>

Usage in template:

<?php

global $post;

$page_id = $post->ID;

$root_parent_id = get_root_parent_id( $page_id );

?>

Another way with native WordPress solution:

<?php

if ( $post->post_parent ) {

	$ancestors = get_post_ancestors( $post->ID );

	$root = count( $ancestors ) - 1;

	$parent = $ancestors[$root];

} else {

	$parent = $post->ID;

}

?>

Another example:

<?php

global $post;

$page_id = $post->ID;

//Here is the Root Parent page

$root_parent_id = get_root_parent_id($page_id);

$root_parent_page = get_post($root_parent_id);

//Here is the page title

$root_parent_title = $root_parent_page->post_title;

//Here is the page link

$root_parent_link = get_permalink($root_parent_id);

?>

<h3><a href="<?php echo $root_parent_link;?>" title="<?php echo $root_parent_title;?>"><?php echo $root_parent_title;?></a></h3>

<ul class="sidebar_menu">

	<?php wp_list_pages("title_li=&child_of=$root_parent_id"); ?>

</ul>