What could be the reason that the PHP session is not functioning?

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There could be various reasons why PHP session is not functioning, some of the common reasons are:

1. Improper Configuration: The session function needs proper configuration to work efficiently. The session function needs the session save path to be set to the correct folder on the server, with proper read and write permissions. If there is any misconfiguration, it may lead to session issues.

2. Server Issues: The cause of session issues could be related to the server on which the application is running. The server may not be running the latest version of PHP, or there may be issues with the server's configuration.

3. Cookies: Session data is stored using cookies, and if cookies are disabled on the client-side, it may cause session issues as the session data won't be stored.

4. Expired Session: If the session has expired, it may cause session issues. It is important to set the correct session timeout value to avoid any expired session issues.

5. Multiple Domain Issues: If the web application is running across multiple domains, it might cause session issues. The session is domain-specific, and if it changes the domain, it may cause problems.

6. Browser Cache: The incorrect server or browser configuration may lead to problems with the session. It is important to clear the browser cache and cookies to avoid any issues with the session.