Description:
------------
The php://input stream behaves differently from other standard streams. In particular, seeking is completely broken. Basically the stream works only when used forward-only, consume-once (eg: file_get_contents('php://input')).

I didn't found the bug reported here (I searched under streams related bugs using the 'seek' keyword). 

I sketched a possible fix with tests here: https://github.com/php/php-src/compare/PHP-7.1...davide-romanini:fix-php-stream-input-seek

Test script:
---------------
--POST--
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Vestibulum ante ex, placerat nec sollicitudin vitae, ultricies molestie nisi. Maecenas lacus magna, augue.
--FILE--
<?php
$i = fopen('php://input', 'r');
fseek($i, 5);
var_dump(fread($i, 10));
rewind($i);
var_dump(fread($i, 5));
fseek($i, -6, SEEK_END);
var_dump(fread($i, 6));
?>
--EXPECT--
string(10) " ipsum dol"
string(5) "Lorem"
string(6) "augue."