Restricting file access using realpath

realpath()

<?php

    $file = pathinfo($_GET['form']);
    $file = realpath($file['dirname']) . DIRECTORY_SEPARATOR . $file['basename'];

    if (empty($file))
    {
        die("No file requested.");
    }

    $invalid = !(preg_match('/^' . preg_quote(realpath('admin/upload')) . '/', $file));
    if ($invalid)
    {
        die("Invalid file path for {$file}");
    }
    
    if (!file_exists($file))
    {
        die("Could not find file {$file}.");
    }