function get_str($id = 0) {
    global $str;
    global $mysqli;
    $sql = "select * from site_nav where nav_pid= $id order by nav_sort";
    $result = mysqli_query($mysqli,$sql);//查询pid的子类的分类
    
    if($result && mysqli_affected_rows($mysqli)){//如果有子类
        $str .= '<ul>';
        while ($row = mysqli_fetch_array($result)) { //循环记录集
            $str .= "<li>" . $row['id'] . "--" . $row['nav_title'] . "</li>"; //构建字符串
            get_str($row['id']); //调用get_str()，将记录集中的id参数传入函数中，继续查询下级
        }
        $str .= '</ul>';
    }
    return $str;
} 
echo get_str(0);