public function getUserIP() {
    $ipaddress = '';
    if (isset($_SERVER['HTTP_CLIENT_IP']) && !empty($_SERVER['HTTP_CLIENT_IP']))
        $ipaddress = $_SERVER['HTTP_CLIENT_IP'];
    elseif(isset($_SERVER['HTTP_X_FORWARDED_FOR']) && !empty($_SERVER['HTTP_X_FORWARDED_FOR']))
        $ipaddress = $_SERVER['HTTP_X_FORWARDED_FOR'];
    elseif(isset($_SERVER['HTTP_X_FORWARDED']) && !empty($_SERVER['HTTP_X_FORWARDED']))
        $ipaddress = $_SERVER['HTTP_X_FORWARDED'];
    elseif(isset($_SERVER['HTTP_X_CLUSTER_CLIENT_IP']) && !empty($_SERVER['HTTP_X_CLUSTER_CLIENT_IP']))
        $ipaddress = $_SERVER['HTTP_X_CLUSTER_CLIENT_IP'];
    elseif(isset($_SERVER['HTTP_FORWARDED_FOR']) && !empty($_SERVER['HTTP_FORWARDED_FOR']))
        $ipaddress = $_SERVER['HTTP_FORWARDED_FOR'];
    elseif(isset($_SERVER['HTTP_FORWARDED']) && !empty($_SERVER['HTTP_FORWARDED']))
        $ipaddress = $_SERVER['HTTP_FORWARDED'];
    elseif(isset($_SERVER['REMOTE_ADDR']) && !empty($_SERVER['REMOTE_ADDR']))
        $ipaddress = $_SERVER['REMOTE_ADDR'];
    else
        $ipaddress = 'UNKNOWN';
    return $ipaddress;
}

It checks various server variables to find the IP address. Here's a breakdown of what it does:

• It initializes the $ipaddress variable to an empty string.
• It checks several server variables ($_SERVER) in a specific order to find the IP address:
  • HTTP_CLIENT_IP
  • HTTP_X_FORWARDED_FOR
  • HTTP_X_FORWARDED
  • HTTP_X_CLUSTER_CLIENT_IP
  • HTTP_FORWARDED_FOR
  • HTTP_FORWARDED
  • REMOTE_ADDR
• If any of these variables are set, it assigns their values to the $ipaddress variable.
• If none of the above variables are set, it assigns 'UNKNOWN' to the $ipaddress variable.
• Finally, it returns the IP address.

This function is useful for applications that need to log or track user IP addresses. However, it's worth noting that relying solely on client-reported IP addresses (which can be manipulated or masked) might not always provide accurate results, especially if used for security purposes.