php errordocument redirect access post

Question

I am using errordocument in .htaccess to redirect a certain not found case to my processing script. This particular not found contains post data, but I cannot access it in my processing script. The content-length header is set, but the data is not available in $_POST or in php://input.

Using liveheaders, I can see that there are only two requests:

POST /page-that-does-not-exist.php HTTP/1.1
Content-Type: application/x-www-form-urlencoded
Content-Length: 9
name=test

HTTP/1.x 200 OK

Any ideas?