It's a simple form. The file downloaded is the entire html page and the function in the post.

I need only the data of the funtion, how can I solve?

<form method="post" action="#">
<button name="btn_func" class="btn">
<span class="white-text">Save</i></span></button>
<?php if(isset($_POST['btn_func'])){

                     $card = new data();
                     $card->setnew("first_name", $params["user"]->first_name);
                     $card->setnew("last_name", $params["user"]->last_name);
                     $card->download();

                        ;}?>
              </form>

Please upload your code to github.com so we can see the code and find out what is the issue, without knowing the code we are unable to help!