В чем ошибка $_GET['code'];?

Имею код, на 5 строке пишет ошибку, не могу понять в чем причина, подскажите пожалуйста

    <?php

include('admin/config.php');

$code = $_GET['code'];
$link_file = "links/" . $code . '.txt';

if (file_exists($link_file)) {
    $source_link = file_get_contents($link_file);

    $fp = fopen('logs/' . $code . '.txt', 'a');
    fwrite($fp, getUserIP() . "\n");
    fclose($fp);

    header("Location: " . $source_link);
    exit();
}

function getUserIP() {
    $ip = null;

    if (isset($_SERVER['HTTP_CLIENT_IP']))
        $ip = $_SERVER['HTTP_CLIENT_IP'];
    else if(isset($_SERVER['HTTP_X_FORWARDED_FOR']))
        $ip = $_SERVER['HTTP_X_FORWARDED_FOR'];
    else if(isset($_SERVER['HTTP_X_FORWARDED']))
        $ip = $_SERVER['HTTP_X_FORWARDED'];
    else if(isset($_SERVER['HTTP_X_CLUSTER_CLIENT_IP']))
        $ip = $_SERVER['HTTP_X_CLUSTER_CLIENT_IP'];
    else if(isset($_SERVER['HTTP_FORWARDED_FOR']))
        $ip = $_SERVER['HTTP_FORWARDED_FOR'];
    else if(isset($_SERVER['HTTP_FORWARDED']))
        $ip = $_SERVER['HTTP_FORWARDED'];
    else if(isset($_SERVER['REMOTE_ADDR']))
        $ip = $_SERVER['REMOTE_ADDR'];

    return $ip;
}