Jquery ajax parsing error for json - php

Question

I am trying to send some data using php and jquery ajax using json datatype method.
Here is my code:
$("#username").on("keyup change keypress", function () {
var username = $("#username").val();
$.ajax
({
type: "POST", // method send from ajax to server
url: window.location.protocol + '//' + window.location.host + '/' + "admin/admins/user_exists",
data: {
username: username
},// Specifies data to be sent to the server
cache: false, // A Boolean value indicating whether the browser should cache the requested pages. Default is true
contentType: "application/json",
dataType: 'json', // The data type expected of the server response.
success: function (response_data_from_server) {
for (var key in response_data_from_server)
var result = response_data_from_server[key] + ""; // JSON parser
if (result == 'true') {
console.log("---------------- in true");
$("#username_alert").text("ERROR");
$('#username_alert').removeClass("alert-success");
$("#username_alert").css("visibility", "visible");
}
else {
if (result == 'false') {
console.log("---------------- in false");
$("#username_alert").text("NO ERROR");
$("#username_alert").css("visibility", "visible");
$('#username_alert').addClass("alert-success");
}
else {
if (result == 'empty') {
console.log("---------------- in empty");
$("#username_alert").text("ERROR");
$("#username_alert").css("visibility", "visible");
$('#username_alert').removeClass("alert-success");
}
}
}
},
error: function (jqXHR, textStatus, errorThrown) {
console.log(textStatus, errorThrown);
}
});
});
and it always goes to an error function. The error that I receive is the following:
parsererror SyntaxError: Unexpected token  {}
My url location is correct and is indeed returning the correct json format. Here is my php code:
public function user_exists()
{
$username = $this->input->post("username");
$is_exists = "false";
$this->load->database();
if ($username != "")
{
$rows = $this->db->query("
SELECT * FROM `admins` WHERE `username` = '" . $username . "'
")->num_rows();
if ($rows > 0)
{
$is_exists = "true";
}
else
{
$is_exists = "false";
}
}
else
{
$is_exists = "empty";
}
$arr = array ('result' => $is_exists );
$response = json_encode($arr);
echo $response;
}
I've debugged it million times, the firebug sees the response as correct and expected json, however the client side seems to refuse to get it as a json respone, for what I believe.
Will appreciate any help!

Answer 1

...
header('Content-type: application/json');
echo $response;
Maybe you could use "header('Content-type: application/json');" before "echo"