new \Twig_SimpleFunction('remote_file', array($this, 'getRemoteFile')), // <- 追加

public function getRemoteFile($url)
{
    $arrContextOptions=array(
        "ssl"=>array(
            "verify_peer"=>false,
            "verify_peer_name"=>false,
        ),
    );
    $response = file_get_contents($url, false, stream_context_create($arrContextOptions));

    echo $response;
    return null;
}

{{  remote_file('https://www.yahoo.co.jp') }}