selenium如何向下滚动页面

Question

我尝试使用selenium webdriver 2.37使用下面的代码向下滚动网页，但它似乎无法正常工作。

<?php
require_once('lib/__init__.php');
    $host = 'http://localhost:4444/wd/hub'; // this is the default
    $capabilities = array(WebDriverCapabilityType::BROWSER_NAME => 'firefox');
    $driver = RemoteWebDriver::create($host, $capabilities,1000);
    $driver->get('https://twitter.com/hashtag/DF14?src=hash');          
    $driver->getKeyboard()->pressKey(WebDriverKeys::ARROW_DOWN);
    sleep(rand(5,7));
?>