/**
     * 图片格式转换
     * @param string $image_path 文件路径或url
     * @param string $to_ext 待转格式，支持png,gif,jpeg,wbmp,webp,xbm
     * @param null|string $save_path 存储路径，null则返回二进制内容，string则返回true|false
     * @return boolean|string $save_path是null则返回二进制内容，是string则返回true|false
     * @throws Exception
     * @author klinson <klinson@163.com>
     */
    function transform_image($image_path, $to_ext = 'png', $save_path = null)
    {
        if (!in_array($to_ext, ['png', 'gif', 'jpeg', 'wbmp', 'webp', 'xbm', 'jpg'])) {
            throw new \Exception('unsupport transform image to ' . $to_ext);
        }

        switch (exif_imagetype($image_path)) {
            case IMAGETYPE_GIF :
                $img = imagecreatefromgif($image_path);
                break;
            case IMAGETYPE_JPEG :
            case IMAGETYPE_JPEG2000:
                $img = imagecreatefromjpeg($image_path);
                break;
            case IMAGETYPE_PNG:
                $img = imagecreatefrompng($image_path);
                break;
            case IMAGETYPE_BMP:
            case IMAGETYPE_WBMP:
                $img = imagecreatefromwbmp($image_path);
                break;
            case IMAGETYPE_XBM:
                $img = imagecreatefromxbm($image_path);
                break;
            case IMAGETYPE_WEBP: //(从 PHP 7.1.0 开始支持)
                $img = imagecreatefromwebp($image_path);
                break;
            default :
                throw new \Exception('Invalid image type');
        }
        $function = 'image' . $to_ext;
        if ($save_path) {
            return $function($img, $save_path);
        } else {
            $tmp = __DIR__ . '/' . uniqid() . '.' . $to_ext;
            if ($function($img, $tmp)) {
                $content = file_get_contents($tmp);
                unlink($tmp);
                return $content;
            } else {
                unlink($tmp);
                throw new \Exception('the file ' . $tmp . ' can not write');
            }
        }
    }