Think/Image/Driver/Gd.php
private function gifNext()
    {
        ob_start();
        ob_implicit_flush(0);
        imagegif($this->img);
        $_img = ob_get_clean();

        $this->gif->image($_img);
        $next = $this->gif->nextImage();

        if ($next) {
            $this->img = imagecreatefromstring($next);

            return $next;
        } else {
            $this->img = imagecreatefromstring($this->gif->image());

            return false;
        }
    }
private function gifNext()
    {
        ob_start();
        ob_implicit_flush(0);
        imagegif($this->img);
        $_img = ob_get_clean();

        $this->gif->image($_img);
        $next = $this->gif->nextImage();

        if ($next) {
            $this->img = imagecreatefromstring($next);

            return $next;
        } else {
            $this->img = imagecreatefromstring($this->gif->image());

            return false;
        }
    }
切换到GIF的下一帧并保存当前帧，内部使用
@return bool