Php function return empty json to Ajax - php

Question

I use ajax to check if it's the first time that the user logs in:
$.ajax({
url: '/checkFirstLogin.php',
type: 'post',
dataType: 'json',
data: {'user_id': userId},
success: function(data) {
if(data == 'firstTime') {
showWelcome();//this open a popup
}else{
alert('been here before');
}
},//end success
}); // end ajax call
checkFirstLogin.php simply does this:
<?php require 'core/init.php';
$user_id = filter_var($_POST['user_id'], FILTER_SANITIZE_NUMBER_INT);
$myUser = new User($user_id);
$myUser->checkFirstLogin();
if($myUser){
$response = 'firstTime';
echo json_encode($response);
}else{
$response = 'beenHere';
echo json_encode($response);
}
User::checkFirstLogin():
public function checkFirstLogin(){
$sth = $this->_db->prepare("SELECT COUNT(user_id) FROM users WHERE first_login = '0' AND user_id= ? ");
$sth->bindParam(1, $this->data()->user_id, PDO::PARAM_INT);
$sth->execute();
$data_exists = ($sth->fetchColumn() > 0) ? true : false;
return $data_exists;
}
json response is always "firstTime" even when first_time = 1 in the database.

Answer 1

You're checking $myUser, not the actual return value of the function; what you mean to do is probably something like;
$is_new_user = $myUser->checkFirstLogin();
if($is_new_user) {
$response = 'firstTime';
...