Ajax SyntaxError：JSON.parse：JSON数据的第1行第1列的意外字符

Ok people. I got this error and i tried all the solutions people with the same problem used to solve it. Nothing. I need to use jQuery "ajax" function to get some data from a php and then distribute the data in some inputs. If i use 'text' as dataType it works like a charm, but with the json dataType i get this error:

SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data

My code is:

js

$.ajax({
            type: 'get',
        url: globals.rootdir + '/controller.php?op=do_something&module=56',
        dataType: 'json',
        data: {
            year: previouslyDeclaredYearVar
        },

            success: function (data) {

                alert('good');

        },
            error: function (xhr, ajaxOptions, thrownError) {
                alert(xhr.status);
                alert(thrownError);

            }
    });

php - the controller do_something function

if (!isAjaxRequest()){
    return;
  }
    header("Content-type: application/json");
    ... do things
    echo json_encode(['result'=>'test-ajax-ok']);