php - laravel base_convert（）无效

I am writing a controller where i have used base_convert function. But instead of converting it, it shows only input value not converted value in output.

Eg: for the below controller it's showing only the $currentID, not the converted one. Why is this happening? If anyone find the problem, please share it with us.

public function getShortUrl(Request $request)
    {
        $this->validate($request,[                      
            'url' => 'required|unique:urls'                             
            ]);
        $url = new Url();
        $url->url = $request->Input(['url']);  
        $url->save();
        $currentId = $url->id;
        $url_code=base_convert($currentId,16,8);
        $url->code ='http://go.'.$url_code;
        $url->save();
    //  return redirect('getURL');
    }