我很难找出代码问题的原因，也就是说，它不会查询“update”部分，但“select”部分确实有效。当我尝试使用print\r函数时，它会给出一个错误/警告，即：
“警告：mysqli\u query（）：无法获取mysqli”和“警告：mysqli\u fetch\u assoc（）期望参数1是mysqli\u result，给定null”

if(!isset($_POST['n_pass'])&&!isset($_POST['n_pass'])){

    if(!isset($_POST['password'])||$_POST['password']==""){
            echo 'enter current password';
            die;
    } else {
            include 'include/database.php';
            $fname = mysqli_real_escape_string($conn, $_POST['fname']);
            $lname = mysqli_real_escape_string($conn,$_POST['lname']);
            $email = mysqli_real_escape_string($conn,$_POST['email']);
            $username = mysqli_real_escape_string($conn,$_POST['uname']);
            $password = mysqli_real_escape_string($conn,$_POST['password']);

            //Check if the password is equal to the password inside database
            $sql = "SELECT password FROM users where id = $id";
            $result = mysqli_query($conn, $sql);
            $row = mysqli_fetch_assoc($result);
            $cpass = $row['password'];
            $verify_pass = password_verify($password,$cpass); //check if current password is equal to the existing password

            if($verify_pass != 1){
                echo 'incorrect password';
                die;
            } else {
              **//Update Data
                $sql="UPDATE users SET firstname=$fname, lastname=$lname, email=$email, username=$username where id=$id";
                $result = mysqli_query($conn, $sql);
                $row = mysqli_fetch_assoc($result);

                print_r($row['firstname']);
                die;
                header("Location: profile.php?successfullyupdated");
            }
        }   
    }