ajax returning white

I once again here. Let's go.

Well, my problem is this:

I am passing values to my method in php via ajax, but they are not returning anything to me, I did some testing and it is working on the next line

I'm going to post the test code (where it returns me) and what I want, however, is catching

  

PHP method

public function listarCidade($id)
{
    $obj_cidade = $this->verTodos("est_id=$id");
    if( $obj_cidade )
    {
        $opt="";
        foreach ($obj_cidade as $key)
            $opt .= "<option value='{$key->id}'>{$key->cid_nome}</option>";

        echo json_encode($opt);
    }
}

In this line:

$opt .= "<option value='{$key->id}'>{$key->cid_nome}</option>";

 -O problema está sendo apenas isso ( '$key->cid_nome' )

Because if I remove this little code it gives me the return but with nothing written in

<option></option>

What to do in galera?