WordPress at a glance
function is not described

wp_get_post_parent_id() WP 3.1.0

Return the post's parent's post_ID

No Hooks.

Return

Int/false. Post parent ID, otherwise false.

Usage

wp_get_post_parent_id( $post_ID );
$post_ID(int) (required)

Code of wp get post parent id: wp-includes/post.php VER 5.0.3

<?php
function wp_get_post_parent_id( $post_ID ) {
	$post = get_post( $post_ID );
	if ( !$post || is_wp_error( $post ) )
		return false;
	return (int) $post->post_parent;
}