代码：

<?php
    $db=mysqli_connect("localhost","root","","mybbs");
    $title=$_POST['title'];
    $content=$_POST['content'];
    $post_time=date('Y-m-d H:i:s');
    $poster=$_SESSION['username'];
    $sql="INSERT INTO `forum` (title,content,poster,post_time) VALUES ('$title','$content','$poster','$post_time')";
    if(mysqli_query($db,$sql)){
        echo "发帖成功！";
    }else{
        echo "发帖失败：".mysqli_error($db);
    }
?>

报错信息：

Warning: Undefined global variable $_SESSION in C:\Users\c\Desktop\Xampp\htdocs\blog\forum_check.php on line 6

Warning: Trying to access array offset on value of type null in C:\Users\c\Desktop\Xampp\htdocs\blog\forum_check.php on line 6

$_SESSION 存过了，其他地方可以正常调用