Как добавить блок со списком подкатегорий, при нахождении в категории?

<?php
if( ! defined( 'DATALIFEENGINE' ) ) { die( "Hacking attempt!" ); }
function CategoryMenu($categoryid = 0, $parentid = 0, $returnstring = '') {
    global $cat_info, $user_group, $member_id;
    $allow_list = explode( ',', $user_group[$member_id['user_group']]['cat_allow_addnews'] );
    $root_category = array ();  
    if( count( $cat_info ) ) {    
        foreach ( $cat_info as $cats ) {
            if( $cats['parentid'] == $parentid ) $root_category[] = $cats['id'];
        }     
        if( count( $root_category ) ) {
            foreach ( $root_category as $id ) {               
                if( $allow_list[0] == "all" or in_array( $id, $allow_list ) ) {           
                    $returnstring .= "<a href='/".$cat_info[$id]['alt_name']."/ title=\"".$cat_info[$id]['descr']."\"'>" . $cat_info[$id]['name']."</a><br>";
                }
                $returnstring = CategoryMenu( $categoryid, $id, $returnstring );
            }
        }
    }
    return $returnstring;
}
echo CategoryMenu(0, $category_id);
?>