google has lots of results.
you could use sessions:
session_start();
if (!isset($_SESSION['lang'])) $_SESSION['lang'] = 'eng';
if (isset($_GET['lang'])) $_SESSION['lang'] = $_GET['lang'];
echo "<a href='?lang=eng'>eng</a> | <a href='?lang=span'>span</a> <br />
(lang = {$_SESSION['lang']})";
then dependent on how you want to load the content, ie; from a database or text file, just use the session somthing like
// if trying to include ie; eng_homepage.html -or- span_homepage.html
include($_SESSION['lang'].'_homepage.html');
// or alternatively, if each db entry has 2 lang's from database
echo $row[$_SESSION['lang'].'_body'];