Function to get user ip address

Well, your function should behave as expected, but here are some suggestions:

// lowercase first letter of functions. It is more standard for PHP
function getIP() 
{
    // populate a local variable to avoid extra function calls.
    // NOTE: use of getenv is not as common as use of $_SERVER.
    //       because of this use of $_SERVER is recommended, but 
    //       for consistency, I'll use getenv below
    $tmp = getenv("HTTP_CLIENT_IP");
    // you DON'T want the HTTP_CLIENT_ID to equal unknown. That said, I don't
    // believe it ever will (same for all below)
    if ( $tmp && !strcasecmp( $tmp, "unknown"))
        return $tmp;
        
    $tmp = getenv("HTTP_X_FORWARDED_FOR");
    if( $tmp && !strcasecmp( $tmp, "unknown"))
        return $tmp;
        
    // no sense in testing SERVER after this. 
    // $_SERVER[ 'REMOTE_ADDR' ] == gentenv( 'REMOTE_ADDR' );
    $tmp = getenv("REMOTE_ADDR");
    if($tmp && !strcasecmp($tmp, "unknown"))
        return $tmp;
        
    return("unknown");
}

Adapted from this answer:

function GetIP()
{
    foreach (array('HTTP_CLIENT_IP', 'HTTP_X_FORWARDED_FOR', 'HTTP_X_FORWARDED', 'HTTP_X_CLUSTER_CLIENT_IP', 'HTTP_FORWARDED_FOR', 'HTTP_FORWARDED', 'REMOTE_ADDR') as $key)
    {
        if (array_key_exists($key, $_SERVER) === true)
        {
            foreach (array_map('trim', explode(',', $_SERVER[$key])) as $ip)
            {
                if (filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_NO_PRIV_RANGE | FILTER_FLAG_NO_RES_RANGE) !== false)
                {
                    return $ip;
                }
            }
        }
    }
}

Checks for IPs (in order) in:

1. HTTP_CLIENT_IP
2. HTTP_X_FORWARDED_FOR
3. HTTP_X_FORWARDED
4. HTTP_X_CLUSTER_CLIENT_IP
5. HTTP_FORWARDED_FOR
6. HTTP_FORWARDED
7. REMOTE_ADDR

Remember that the only IP address you can trust is the one coming from $_SERVER['REMOTE_ADDR'].