Php 如何获取插入行的ID并更新行？

我想在我的表中获取ID，然后更新 id行是自动递增的

if (isset($_POST['reply'])) {
    $reply = $_POST['reply'];
    $answers = $db->query("SELECT * FROM table_name");
    while($answers_ = $answers->fetch_object()){
        if($get_answer = $db->query("UPDATE table_name SET answer ='$reply' WHERE question_id = '$answers_->question_id' LIMIT 1")){
            echo "done";    
        }
    }
}   

我知道我的代码是错误的，它使用相同的值更新所有行我只想更新一行

$get_answer = $db->query("UPDATE table_name SET answer ='$reply' WHERE question_id = '$answers_->question_id' LIMIT 1")

$last_id = mysql_insert_id();