USING a variable in include

Here is a code that i tried to design but it wornd work.. can someone plz tell me what m i doing wrong?

<?php 
$folder=$_GET['folder'];
include ('.$folder.'/main'.php');
 ?>

PHP:

 

<?php 
    if(isset($_GET['folder']) && !empty($_GET['folder'])){
        $folder= $_GET['folder'];
        include ($folder . "/main.php");
    }else{
        echo "Hello there!";
    }
?>

PHP:

Note: Make sure to verify the $_GET[] values and make it open only certain pages/folders or otherwise you'll make hackers access other folders/files

Thanks

 

Assuming you have ?folder=something

<?php
$folder=$_GET['folder'];
include ("$folder/main.php");

 ?>

PHP:

Or as above run a check