What are some common mistakes or miscalculations that can lead to unexpected session timeouts in PHP scripts?

Common mistakes that can lead to unexpected session timeouts in PHP scripts include not setting a long enough session timeout value, not properly handling session regeneration, and not checking for session expiration before accessing session variables. To solve these issues, make sure to set a sufficient session timeout value using the `session_set_cookie_params()` function, regenerate the session ID periodically using `session_regenerate_id()`, and check if the session has expired using `session_status()` before accessing session variables.

// Set a longer session timeout value
session_set_cookie_params(3600); // 1 hour

// Regenerate session ID periodically
if (time() - $_SESSION['last_activity'] > 1800) { // 30 minutes
    session_regenerate_id(true);
    $_SESSION['last_activity'] = time();
}

// Check if session has expired before accessing session variables
if (session_status() === PHP_SESSION_ACTIVE && time() - $_SESSION['last_activity'] < 3600) { // 1 hour
    // Access session variables here
} else {
    // Session has expired, handle accordingly
}