[HELP] Cách tạo session làm web đa ngôn ngữ trong PHP

<?php
if (isset($_GET['lang'])){  // check GET trước
   $lang=strtolower($_GET['lang']);
   $_SESSION['lang']=$lang;
}elseif (isset($_SESSION['lang'])){
   $lang=strtolower($_SESSION['lang']);
}else{
   $lang="vi"; // default language
   $_SESSION['lang']="vi";
}
if (file_exists("lang/$lang.php")){
       include("lang/$lang.php");
}else{
// lang not exist, include default
       include("lang/vi.php");
       $_SESSION['lang']="vi";
}
?>

<?php
function curPageURL() {
 $pageURL = 'http';
 if ($_SERVER["HTTPS"] == "on") {$pageURL .= "s";}
 $pageURL .= "://";
 if ($_SERVER["SERVER_PORT"] != "80") {
  $pageURL .= $_SERVER["SERVER_NAME"].":".$_SERVER["SERVER_PORT"].$_SERVER["REQUEST_URI"];
 } else {
  $pageURL .= $_SERVER["SERVER_NAME"].$_SERVER["REQUEST_URI"];
 }
 return $pageURL;
}
?>

<?php 
echo '<a href="'.curPageURL().'&lang=vi">Tiếng Việt</a><br />'; 
echo '<a href="'.curPageURL().'&lang=en">English</a><br />'; 
?>

http://localhost/webphp/&lang=en

http://localhost/webphp/&lang=vi

$curpage=curPageURL();
if (stripos($curpage,"?")===false){  // không tìm thấy
     $link="$curpage?lang=";
}else{
     $link="curpage&lang=";   
}
echo '<a href="'.$link.'vi">Tiếng Việt</a>';
echo '<a href="'.$link.'en">English</a>';
echo '<a href="'.$link.'fr">France</a>';