parsererror / SyntaxError: Unexpected token <

I've searched a lot for this but the answers that I find doesn't work but I think that the problem is mine.

JavaScript:

$.ajax({
    type: 'POST',
    url: url,
    data: data,
    dataType: 'json',
    success: function(response) {
        if(response.status == true) {
            alert('ok');
        } else {
            alert('error');
        }
    },
    error: function(xhr, desc, err) {
        console.log(xhr);
        console.log("Details: " + desc + "\nError:" + err);
    }
});

PHP

if(User::addFavLater($id, $user, 'favs')) {
    $result = array("status" => true);
} else {
    $result = array("status" => false);
}

header('Content-type: application/json');
echo json_encode($result);

So, this is an excerpt of code that I have and I think can you realising. I want PHP to send a JSON response into JavaScript but this is not happening and JavaScript gives me this error:

parsererror / SyntaxError: Unexpected token <

I'm using the 2.1.1 version of JQuery.

P.S.: Sorry for my english.