网站本地XAMPP运行正常，部署免费主机后报错求助

Hey there! Let's break down these two common PHP errors and get your site working smoothly on the free host—no stress, these are super fixable.

1. Notice: Undefined index: login_user (session.php line 6) #

What's causing it? #

This pops up because you're trying to access $_SESSION['login_user'] without first checking if that session variable actually exists. Your local XAMPP probably hides these minor notices by default, but free hosts tend to show all error types. When a user isn't logged in, that session key doesn't exist yet—hence the "undefined index" warning.

How to fix it: #

Always validate that the session variable exists before using it with isset() or empty(). Here's a cleaned-up version of your session.php:

<?php
// Critical: start the session FIRST, before any output!
session_start();

// Check if the user is logged in
if (!isset($_SESSION['login_user'])) {
    // Redirect to login page if no active session
    header("Location: login.php");
    exit(); // Stop the script immediately to avoid extra output
}
// Rest of your logged-in-only logic goes here
?>

2. Warning: Cannot modify header information - headers already sent... #

What's causing it? #

PHP's header() function needs to send HTTP headers before any content reaches the browser. This error happens if there's even a single space, line break, HTML tag, or echo statement before your session_start() or header() calls. Free hosts are often stricter about this than local XAMPP setups.

Common culprits:

• A space/empty line before the <?php opening tag in session.php (or any included file)
• Accidental echo/print debug statements before the redirect
• Your text editor saved the file with a hidden UTF-8 BOM character at the start

How to fix it: #

• Check file openings: Ensure every PHP file involved (session.php, dashboard.php, login.php) starts with <?php as the very first character—no spaces, no newlines, nothing before it.
• Eliminate early output: Remove any echo statements, HTML code, or whitespace that comes before session_start() or header().
• Fix file encoding: If you use editors like Notepad++, save your files with "UTF-8 without BOM" encoding (look in the encoding menu).
• Stick with exit() after redirects: As shown in the code above, exit() ensures the script stops right after sending the redirect, so no extra content slips through.

Quick Local Development Tip #

To catch these errors earlier (before uploading to the host), add this to the top of your PHP files during testing:

error_reporting(E_ALL);
ini_set('display_errors', 1);

This mirrors the stricter error reporting of most free hosts on your local XAMPP setup.

内容的提问来源于stack exchange，提问作者Justine Noob