What are the security implications of using $_SERVER["REMOTE_ADDR"] to track unique visitors in PHP?

Using $_SERVER["REMOTE_ADDR"] to track unique visitors in PHP can be unreliable as it can be easily spoofed or manipulated by users using proxies or VPNs. To improve security, it's recommended to use a combination of $_SERVER["REMOTE_ADDR"] along with other server variables like HTTP_USER_AGENT to create a more robust visitor tracking system.

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