How to use "?main=" in HTML / PHP

I am trying to reference a file from a subdirectory of my root folder in a way that only the "main" changes.

I have this pice of code in my home.php file:

<div id="main" class="main">
        <?php require $_GET['main'] . ".php"; ?>
    </div>

All the navigation buttons should change the "main" to certain files in various folders under the folder which contains home.php.

The below code snippet works perfectly if the file I am referencing is in the same folder as my working file:

<a href="?main=dashboard.php">Dashboard 1</a>

What I want to do is (I imagine) like this:

<a href="?main=folder/dashboard.php"> Dashboard 1</a>

where the file I am referencing is in a different folder.

Also, if I reference the file in question with

<a href="folder/dashboard.php"> Dashboard 1</a>

the file loads, but it does not target the "main" like it should with ?main=

I tried myriad ways that I read about / though would work, but nothing did so far. Any help would be much appreciated.

Solution

Don't require files from user input without validation!

<?php
// For Security a list of granted paths.
$allowedPaths = [
    'folder/dashboard.php',
];

$main = isset($_GET['main']) ? $_GET['main'] : '';
if(!in_array($main, $allowedPaths) {
   // Error handling
   die('Path not allowed');
}

require $main;
?>

The link could look like

<a href="?main=folder%2Fdashboard.php">

Note, that the URL values must be encoded -> echo urlencode('folder/dashboard.php'); to make the slash valid.
Note to not add .php extension twice (once in URL and again in code).