RESTful API with Volley Kotlin doens't work - php

Question

I've seen many others questions and answers on the Web, but I haven't found the solution to my issue.
Basically, I've create an API page. If I use a API tester (such as apitester.com) it works correctly.
Instead, when I do POST request from Volley (Android-Kotlin) it doens't work.
It looks like params are empty.
I can't understand the problem.
PHP Code:
include_once($_SERVER['DOCUMENT_ROOT'] . "/include/variables.php");
global $localhost_db, $username_db, $password_db, $database_api;
header("Content-Type:application/json");
if ($c = new mysqli($localhost_db, $username_db, $password_db, $database_api)) {
$c->set_charset("utf8");
//POST request -> insert a new data to database
$data = json_decode(file_get_contents('php://input'), true);
$condition = isset($data["logged"]) && ($data["logged"] == 0 || $data["logged"] == 1) && isset($data["username"]) && isset($data["language"]);
if ($condition) {
$year = date('Y');
$month = date("m");
$day = date("d");
$sql = "SELECT `date` FROM statistics WHERE `username`='" . $data["username"] . "' AND YEAR(`date`)=" . $year . " AND MONTH(`date`)=" . $month . " AND DAY(`date`)=" . $day;
if ($r = $c->query($sql)) {
if ($r->num_rows == 0) {
$sql = "INSERT INTO statistics(`id`, `date`, `logged`, `username`, `language`) VALUES(NULL,'" . date("Y-m-d H:i:s") . "', '" . $data["logged"] . "', '" . $data["username"] . "', '" . $data["language"] . "')";
if ($r = $c->query($sql)) {
response(200, "OK", "Record inserted correctly");
} else {
response(500, "Error", "Can't insert record on database");
}
} else {
response(400, "Error", "Record has already inserted today");
}
} else {
response(400, "Error", "Something was wrong in POST request (1)");
}
} else {
response(400, "Error", "Something was wrong in POST request (2). Received data> logged: " . $data["logged"] . ", language: " . $data["language"] . ", username: " . $data["username"]);
}
} else {
echo "Failed to connect to the database.";
response(500, "Error", "Can't connect to the database");
}
function response($response_code, $response_status, $response_description)
{
$response['code'] = $response_code;
$response['status'] = $response_status;
$response['description'] = $response_description;
$json_response = json_encode($response);
echo $json_response;
}
?>
instead the Android Kotlin code:
var params = JSONObject()
params.put("username", username)
params.put("logged", logged)
params.put("language", language)
val que = Volley.newRequestQueue(this)
val req = object : JsonObjectRequest(Request.Method.POST, url_statistics, params,
Response.Listener {
val jsonResult = it.toString()
var jsonResultArray = arrayOf(jsonResult, "")
println(jsonResult)
val jsonObj = JSONObject(
jsonResultArray[0].substring(
jsonResultArray[0].indexOf("{"),
jsonResultArray[0].lastIndexOf("}") + 1
)
)
if (jsonObj.getString("code")
.toInt() == 200
) {//Successful}
else {//Error}
}, Response.ErrorListener {
//Error
}
){}
que.add(req)

Answer 1

do this:
$username = $_POST['username']
instead of:
$data = json_decode(file_get_contents('php://input'), true);