How can firewalls affect the functionality of the HTTP_REFERER variable in PHP applications?

Firewalls can sometimes block or modify the HTTP_REFERER variable, which can affect the functionality of PHP applications that rely on this variable for tracking or security purposes. To ensure that the HTTP_REFERER variable is not blocked or modified by firewalls, you can set up your firewall rules to allow the HTTP_REFERER header to pass through without interference.

// Set up firewall rules to allow the HTTP_REFERER header to pass through
header("Referrer-Policy: origin-when-cross-origin");