$username = $_POST['username'];$result = mysqli_query($conn,"SELECT * FROM user Where username = '".username."'");$id = $_GET['id'];$result = mysqli_query($conn,"SELECT body FROM user Where id = $id");以上這兩串代碼中，第一條 中單引號(hào)，雙引號(hào)，和點(diǎn)的用處是什么？還有為什么不像下面的那個(gè)id一樣直接寫上去呢？