What are the potential pitfalls of not handling cookies properly in PHP, especially in scenarios where domain variations affect cookie validity and accessibility?

Improper handling of cookies in PHP can lead to issues with cookie validity and accessibility, especially in scenarios where domain variations come into play. To ensure cookies work correctly across different domains, it's important to set the cookie domain parameter to a common base domain that all variations share. This allows the cookie to be accessible across all subdomains and variations of the domain.

// Set cookie with a common base domain for all variations
setcookie('cookie_name', 'cookie_value', time() + 3600, '/', '.example.com', false, true);