see http://blog.sarabande.jp/post/127807990268

psr7stream.php

 <?php
 use Zend\Diactoros\Response;

 require_once __DIR__.'/vendor/autoload.php';

 $res = new Response();
 $res->getBody()->write('hello');

 $contents = $res->getBody()->getContents();
 var_dump($contents); // ""

 $res->getBody()->seek(0);
 $contents = $res->getBody()->getContents();
 var_dump($contents); // "hello"

 $res->getBody()->seek(0);
 $contents = $res->getBody()->getContents();
 var_dump($contents); // "hello"

 $res->getBody()->seek(2);
 $contents = $res->getBody();
 var_dump((string)$contents); // "hello"

 $contents = $res->getBody()->getContents();
 var_dump($contents); // ""