Ajax Confusing http status - php

Question

I have a register form and I want to show some messages in a div. I use Ajax for this. The confusing fact is that in the ajax block, it enters on 'error' branch and show http status 200. It is ok to do that? The submit event is on the form. Should I put on the button? How can I fix it to do what I want?
<form id="register" class="modall-content animate" action="register.php" method="post">
......
<div id = "error-reg"></div>
<div id = "success-reg"></div>
</form>
Php code is this
if(isset($_POST['btn-rg'])) {
..
if ($count == 0) {
if ($check == 1)
$query = "INSERT INTO ...";
elseif ($check == 2)
$query = "INSERT INTO ...";
else {
$query = "INSERT INTO ...";
}
if ($db->query($query)) {
$success .= "Success";
/*echo $success;*/
echo json_encode(array("success" => $success));
}
} else {
$message .= "Username already exists";
/*echo $message;*/
echo json_encode(array("message" => $message));
}
/*$response = array();
$response['success'] = $success;
$response['errors'] = $message;
echo(json_encode($response));*/
}
And my js
$("#register").on('submit',function (event) {
event.preventDefault();
var dataPost= $('#register').serialize();
$.ajax({
url: 'register.php',
type: 'post',
dataType : 'json',
data: dataPost,
success: function(data) {
if (response.success) {
$('#error-reg').hide();
$('#success-reg').html(response.success).css('color','green');
} else {
$('#error-reg').html(response.errors);
}
},
error: function (data) {
console.log(data);
}
});
});
When I make the submit this is what I get

Answer 1

In your register.php right at the top.
Keep this code, but comment it out.It will give you access to see and make sure what comes in.
// echo json_encode($_POST);
// exit;
You did see $_POST['btn-rg']??? ---> NO !!
Did you declare your HTML button as an input?
<input type='submit' name='btn-rg' value='Submit'>
Now take the folowng lines and put it in top of the if-statement. I want to be sure we get inside this statement. You should expect to see "hello world " again.
echo json_encode(array("message" => "hello world"));
exit;