使用FILTER_SANITIZE_URL解释爆炸功能

I have some problem with Explode function.

public function parseUrl()
    {
        if(isset($_GET['url'])){
          $tmp = filter_var(rtrim($_GET['url'],'/'), FILTER_SANITIZE_URL);
          return $url = explode('/', $tmp);
        }
    }

If i'm using this this isn't working but if i do this

public function parseUrl()
    {
        if(isset($_GET['url'])){
          //$tmp = filter_var(rtrim($_GET['url'],'/'), FILTER_SANITIZE_URL);
          return $url = explode('/', filter_var(rtrim($_GET['url'],'/'), FILTER_SANITIZE_URL));
        }
    }

This works fine can anyone explain what is the problem if I'm using one more variable in Explode function