Untested self-hosted linkr script

linkr.php

<?php

/* linkr self-hosted shortener
 * © aerouk 2014
 */

$key = "your key";
$err = "/path/to/404"; // public_html/*

if ( ! isset($_GET['url'])) showError($err);

checkURL($_GET['url']);

function checkURL($url, $key)
{
    global $err;
    $result = json_decode(file_get_contents("https://api.linkr.pw/v3/lookup?key=$key&short=$url"), true);

    if (isset($result['error'])) {
        showError($err);
    } elseif (isset($result['link'])) {
        header("Location: https://linkr.pw/$url", true, 301);
        die();
    } else {
        showError($err);
    }
}

function showError($err)
{
    @include_once($_SERVER['DOCUMENT_ROOT'] . $err);
    die();
}