登录页面不显示PHP SESSION信息

Question

在user.php中，我有以下几行，当我登录时，SESSION信息在user.php中从validate.php中获取，但一个用户没有登录，我想在login.php中用 "你必须先登录 "信息重定向他们。

<?php
    include('inc/header.php');

    if (isset($_SESSION['username'])) {


    include('lib/config.php');
    define('IMG_PATH', 'https://'.$_SERVER['SERVER_NAME'].'/demo/jobs/images/');

    $user = $_SESSION['username'];
    $sql = "SELECT * FROM candidates WHERE username='$user'";
    //$result = mysqli_result object
    $result = $conn->query( $sql );

    $row = $result->fetch_assoc();
    $imgpath = $row['avatar'];
    $img = substr( strrchr( $imgpath, "/" ), 1) ; 

?>
    <!-- notification message -->
    <?php if (isset($_SESSION['message'])) : ?>
        <div class="alert alert-success" >
            <h3>
                <?php 
                    echo $_SESSION['message']; 
                    unset($_SESSION['message']);
                ?>
            </h3>
        </div>
    <?php endif ?>
    <div class="welcome">
        <div class="profile_card">
            <img class="profile_avatar" src="<?php echo IMG_PATH . $img; ?>">
            <h3 class="profile_name"><?php echo $row['fullname']; ?></h3>
            <div class="profile_contact phone"><i class="material-icons">phone</i><span><?php echo $row['phone']; ?></span></div>
            <div class="profile_contact email"><i class="material-icons">email</i><span><?php echo $row['email']; ?></span></div>
            <p> <a href="logout.php?logout='1'" style="color: red;">logout</a> </p>
        </div>
    </div>
<?php
    } else {

        $_SESSION['message'] = "You must log in first";
        header('location: login.php');
    }
    include('inc/footer.php');
?>

谢谢你花时间来检查代码，并感谢要求登录表单代码清晰的人，我也是login.php脚本，如下所示。

<?php
    // header
    include('inc/header.php');

    include('lib/validate.php');

?>

<div class="login_form">
    <h2>Login your account</h2>
    <form class="form" action="login.php" method="post" autocomplete="off">
        <div class="alert alert-error"><?= $_SESSION['message'] ?></div>
        <div class="username out-group">
            <label for="username">Username or Email:</label>
            <input type="text" placeholder="ashif" name="username" required />
        </div>
        <div class="password out-group">
            <label for="password">Password:</label>
            <input type="password" placeholder="ue$jL382@0l" name="password" required />
        </div>
        <input type="submit" value="Login" name="login" class="btn btn-block btn-primary" />
        <p>
            Not yet a member? <a href="register.php">Join</a>
        </p>
    </form>
</div>

<?php
    include('inc/footer.php');
?>