Source

File: wp-admin/includes/template.php

function parent_dropdown( $default = 0, $parent = 0, $level = 0, $post = null ) {
	global $wpdb;

	$post  = get_post( $post );
	$items = $wpdb->get_results( $wpdb->prepare( "SELECT ID, post_parent, post_title FROM $wpdb->posts WHERE post_parent = %d AND post_type = 'page' ORDER BY menu_order", $parent ) );

	if ( $items ) {
		foreach ( $items as $item ) {
			// A page cannot be its own parent.
			if ( $post && $post->ID && (int) $item->ID === $post->ID ) {
				continue;
			}

			$pad      = str_repeat( ' ', $level * 3 );
			$selected = selected( $default, $item->ID, false );

			echo "\n\t<option class='level-$level' value='$item->ID' $selected>$pad " . esc_html( $item->post_title ) . '</option>';
			parent_dropdown( $default, $item->ID, $level + 1 );
		}
	} else {
		return false;
	}
}