<?php
session_start();
//get referrer url
if (!isset($_SESSION["origURL"]))
    $_SESSION["origURL"] = $_SERVER["HTTP_REFERER"];

$url = $_SESSION["origURL"];

//get the 123/456 from the url
$last = parse_url($url, PHP_URL_PATH);

$where = substr($last, -7, 3);

//get the users ip address
$IP_address = $_SERVER["REMOTE_ADDR"];

//where should we send the user?
if ($IP_address == "123.456.789" || $IP_address == "987.654.321") {
    header( 'Location: http://example.com/original-link' );
}
else {
    if ($where == "123") {
        header( 'Location: http://example.com/custom_string1='.$where ) ;
    }
    elseif ($where == "456") {
        header( 'Location: http://example.com/custom_string2='.$where );
    }
}
?>