Why does include('php://input') not work?

Question

Imagine a toy PHP application vulnerable to absolute local file inclusion, e.g.

<?php include($_GET['action']);

I tried the following request to exploit it:

POST /?action=php://input HTTP/1.1
Host: XXXXXXXXXXXXXXXXX
Content-Length: 3

foo

This effectively executes include('php://input'); with request body foo, so I would expect it to print foo. However, I get the following error

<br />
<b>Warning</b>:  include(php://input): failed to open stream: operation failed in <b>XXXXXXXXXXXXXXXXX</b> on line <b>12</b><br />
<br />
<b>Warning</b>:  include(): Failed opening 'php://input' for inclusion (include_path='.:/usr/share/php:/usr/share/pear') in <b>XXXXXXXXXXXXXXXXXXX</b> on line <b>12</b><br />

What is the issue here? Is this a PHP security feature? If so, can somebody point to the responsible part of the PHP source code that mitigates this?

Answer 1

I found the answer with the help of Gustek. Apparently php://input falls under the restrction of allow_url_include, while for example php://filter does not:

Restricted by allow_url_include: php://input, php://stdin, php://memory and php://temp only.

Source: Docs for php:// URL handler