WordPress is_subpage() function to check if a page against it's immediate parent or all it's parents

is_subpage.php

/**
* Is SubPage?
*
* Checks if the current page is a sub-page and returns true or false.
*
* @param $page mixed optional ( post_name or ID ) to check against.
* @param $single boolean optional - default true to check against all parents, false to check against immediate parent.
* @return boolean
*/

function is_subpage( $page = null, $all = true ) {
  global $post;
	// is this even a page?
	if ( ! is_page() )
		return false;
	// does it have a parent?
	if ( ! isset( $post->post_parent ) OR $post->post_parent <= 0 )
		return false;
	// is there something to check against?
	if ( ! isset( $page ) ) {
		// yup this is a sub-page
		return true;
	} else {
		// if $page is an integer then its a simple check
		if ( is_int( $page ) ) {
			// check
			if ( $post->post_parent == $page )
				return true;
		} else if ( is_string( $page ) ) {
			// get ancestors
			$parents = get_ancestors( $post->ID, 'page' );
			// does it have ancestors?
			if ( empty( $parents ) )
				return false;
			if ( $all == true ) { // check against all parents
				// loop through ancestors
				foreach ( $parents as $parent ) {
					$parent = get_post( $parent );
					if ( is_page() && $parent->post_name == $page) {
						return true;
					}
				}
			} else { // check against immediate parent
				// get the first ancestor
				$parent = get_post( $parents[0] );
				// compare the post_name
				if ( $parent->post_name == $page )
					return true;
			}
		}
		return false;
	}
}