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<?php
/*
 * Function / Action to enable choosing private pages as parent pages
 */
function admin_private_parent_metabox($output)
{
    global $post;

    $args = array(
        'post_type' => $post->post_type,
        'exclude_tree' => $post->ID,
        'selected' => $post->post_parent,
        'name' => 'parent_id',
        'show_option_none' => __('(no parent)'),
        'sort_column' => 'menu_order, post_title',
        'echo' => 0,
        'post_status' => array('publish', 'private'),
    );

    $defaults = array(
        'depth' => 0,
        'child_of' => 0,
        'selected' => 0,
        'echo' => 1,
        'name' => 'page_id',
        'id' => '',
        'show_option_none' => '',
        'show_option_no_change' => '',
        'option_none_value' => '',
    );

    $r = wp_parse_args($args, $defaults);
    extract($r, EXTR_SKIP);

    $pages = get_pages($r);
    $name = esc_attr($name);
// Back-compat with old system where both id and name were based on $name argument
    if (empty($id)) {
        $id = $name;
    }

    if (!empty($pages)) {
        $output = "<select name=\"$name\" id=\"$id\">\n";

        if ($show_option_no_change) {
            $output .= "\t<option value=\"-1\">$show_option_no_change</option>";
        }
        if ($show_option_none) {
            $output .= "\t<option value=\"" . esc_attr($option_none_value) . "\">$show_option_none</option>\n";
        }
        $output .= walk_page_dropdown_tree($pages, $depth, $r);
        $output .= "</select>\n";
    }

    return $output;
}

add_filter('wp_dropdown_pages', 'admin_private_parent_metabox');