How to convert php image type

/**
 * 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 $save_path是null则返回二进制内容，是string则返回true|false
 * @throws Exception
 */
function image_change($image_path, $to_ext = 'png', $save_path = null)
{
    if (!in_array($to_ext, ['png', 'gif', 'jpeg', 'wbmp', 'webp', 'xbm'])) {
        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');
        }
    }
}