processForm.php为我的会话ID提供了零值

Question

我想从另一个表中存储数据（id，profile_image，标题）（我只是下载用于上传图像的代码）问题是当我要保存数据时，processForm.php始终为我提供Session ID的零值，我当前的ID是“ 1”。我是这里的新手。

（login.php）

    <?php

    // Initialize the session

    session_start();

    // Check if the user is already logged in, if yes then redirect him to 

    welcome page

    if(isset($_SESSION["loggedin"]) && $_SESSION["loggedin"] === true){

    header("location: welcome.php");

    exit;

    }

    // Include config file

    require_once "config.php";

    // Define variables and initialize with empty values

    $username = $password = "";

    $username_err = $password_err = "";

    // Processing form data when form is submitted

    if($_SERVER["REQUEST_METHOD"] == "POST"){

    // Check if username is empty

    if(empty(trim($_POST["username"]))){

    $username_err = "Please enter username.";

    } else{

    $username = trim($_POST["username"]);

    }

    // Check if password is empty

    if(empty(trim($_POST["password"]))){

    $password_err = "Please enter your password.";

    } else{

    $password = trim($_POST["password"]);

    }

    // Validate credentials

    if(empty($username_err) && empty($password_err)){

    // Prepare a select statement

    $sql = "SELECT id, username, password FROM users WHERE username = ?";

    if($stmt = mysqli_prepare($link, $sql)){

        // Bind variables to the prepared statement as parameters

        mysqli_stmt_bind_param($stmt, "s", $param_username);

        // Set parameters

        $param_username = $username;