問題常見原因包括未正確啟動會話、重定向前未保存會話數據或會話 cookie 配置不一致。1. 確保每個需要會話數據的 PHP 文件頂部調用 session_start()，且無輸出干擾；2. 重定向前使用 session_write_close() 強制保存會話數據；3. 通過 session_set_cookie_params() 統(tǒng)一設置會話 cookie 的路徑和域名參數，確?？珥撁嬉恢滦?。

You're probably losing your PHP session after a redirect because the session isn't being properly started or maintained across requests. This is a common issue, especially when redirects are involved, and it often comes down to subtle misconfigurations or missed function calls.

Session not started before using or redirecting

One of the most common reasons sessions get lost after a redirect is that session_start() wasn’t called early enough — or at all — in the script. If you try to set or access session data without starting the session first, PHP won’t know what session to use, and the redirect might make it look like everything was lost.

• Always call session_start() at the top of every PHP file where you need session data.
• Make sure there’s no output (like HTML or even whitespace) before session_start(), or else headers will already be sent and session cookies may not work correctly.

For example:

<?php
session_start();
// Your code here
header('Location: next-page.php');
exit;
?>

If you forget session_start() on either this page or next-page.php, the session data won’t carry over.

Redirect happens before session data is saved

PHP doesn’t save session data until the script ends or until you explicitly call session_write_close(). If you trigger a redirect with header('Location: ...') and then immediately exit (which you should), there's a chance the session hasn’t been fully written yet — especially if you’re doing heavy processing before the redirect.

To avoid this:

• Call session_write_close() before redirecting if you're done working with the session.

<?php
session_start();
$_SESSION['message'] = 'Login successful';
session_write_close(); // Ensures session data is saved
header('Location: dashboard.php');
exit;
?>

This helps ensure the session data is safely stored before the browser moves on.

Session cookie path or domain misconfiguration

Another possible reason is that the session cookie path or domain doesn’t match between the original request and the redirected one. For example, if the session cookie is set for /admin, but the redirect goes to /public, the browser won’t send the cookie, and PHP will start a new session instead.

You can check and configure this using session_set_cookie_params() before calling session_start():

session_set_cookie_params([
    'lifetime' => 0,
    'path' => '/',
    'domain' => '', // or your domain
    'secure' => true, // if using HTTPS
    'httponly' => true,
]);
session_start();

Make sure these settings are consistent across all pages involved in the redirect chain.

These issues are easy to overlook but straightforward to fix once you know what to look for. Most of the time, it's just a matter of making sure the session is started early, written before redirecting, and configured to work across the right paths and domains.

基本上就這些。

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