The script below requires a SQL database called customURL.sqlite placed in the same directory as registerURL.php which has the following structure:

BEGIN TRANSACTION;
CREATE TABLE "urls" (
	 "URL" text NOT NULL,
	PRIMARY KEY("URL")
);
COMMIT;

registerURL.php

<?php
 
        //////////////////////////////////////////////////////////
        //     WaS (c) grimore.org - 2012, License: GPLv3            //
        // Please see: http://www.gnu.org/licenses/gpl.html     //
        // for legal details, rights of fair usage and          //
        // the disclaimer and warranty conditions.              //
        //////////////////////////////////////////////////////////
 
        // For privacy, you need to generate a password. You can do that
        // by issuing the following command on an *nix-like system and 
        // replace "super-duper secret password" with a secret shared between
        // this PHP script and the LSL counterpart:
        // sh$ echo "super-duper secret password" | sha1sum  | awk '{ print $1 }'
        $apiKey = 'A1364788-B702-4413-B937-7A57E32B43CE';
 
        if(!isset($_GET['apiKey']) || $_GET['apiKey'] != $apiKey) {
                throw new Exception('Invalid API key.');
                return;         
        }
        if(!isset($_GET['action'])) {
                throw new Exception('No action specified.');
                return;
        }
        switch($_GET['action']) {
                case "register":
                        if(!isset($_GET['llURL']) && !isset($_GET['shortURL'])) return;
                        $llURL = $_GET['llURL'];
                        $shortURL = $_GET['shortURL'];
                        $db = new PDO('sqlite:customURL.sqlite');
                        $db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
                        $q = $db->prepare("REPLACE INTO url_table(llURL, shortURL) VALUES(:llURL, :shortURL)");
                        try {
                                $q->execute(array(':llURL' => $llURL, ':shortURL' => $shortURL));
                        }
                        catch(PDOException $e) {
                                die($e->getMessage());
                        }
                        print $shortURL;
                        break;
                case "retrieve":
                        if(!isset($_GET['shortURL'])) return;
                        $shortURL = $_GET['shortURL'];
                        $db = new PDO('sqlite:customURL.sqlite');
                        $db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
                        $q = $db->prepare("SELECT llURL FROM url_table WHERE shortURL=:shortURL LIMIT 1");
                        try {
                                $q->execute(array(':shortURL' => $shortURL));
                                $res = $q->fetch(PDO::FETCH_OBJ); 
                        }
                        catch(PDOException $e) {
                                die($e->getMessage());
                        }
                        print $res->llURL;
                        break;
                default:
                        break;
        }
?>