What are some potential issues with using IP addresses for user identification in PHP?

One potential issue with using IP addresses for user identification in PHP is that multiple users can share the same IP address, such as in the case of users behind a proxy server. This can lead to incorrect identification and potential security risks. To solve this issue, you can use a combination of IP address and user agent string for more accurate user identification.

$user_identifier = hash('sha256', $_SERVER['REMOTE_ADDR'] . $_SERVER['HTTP_USER_AGENT']);