The catch will never be called, and you should never use isset() because it will return false if the value exists but is null.

    whatever->param = null;
    whatever->param = 123;

    function __set($k, $v){
        if (array_key_exists($k, $this->Properties)){
            throw new ReadOnlyException();
        }else{
            $this->Properties[$k] = $v;
        }
    }