What could be causing the session to display "404.php" due to a subdomain in PHP?

The issue of the session displaying "404.php" due to a subdomain in PHP could be caused by incorrect configuration of the session cookie domain. To solve this issue, you need to set the session cookie domain to the root domain to ensure that the session is shared across all subdomains.

// Set the session cookie domain to the root domain
ini_set('session.cookie_domain', '.yourdomain.com');