How can i check if only one or both checkboxes are checken

I have to check if one checkbox is checked or both. But the problem is, if i try my code. The site has an error ("This site doesn't work").

I tried this:

    $lw1l = $_POST['lw1l'];
    $lw1s = $_POST['lw1s'];
    $lw2l = $_POST['lw2l'];
    $lw2s = $_POST['lw2s'];

    if(isset($lw1l)){
      if(isset($lw1s)){
        $lw1 = "read / write";
      }else{
        $lw1 = "read ";
      }
    }else{
      isset($lw1s){
        $lw1 = "write";
      }
    }

    if(isset($lw2l)){
      if(isset($lw2s)){
        $lw2 = "read/ write";
      }else{
        $lw2 = "read";
      }
    }else{
      if(isset($lw2s)){
        $lw2 = "write";
      }
    }

But this doesn't work...

Would be glad if somebody could help :D