too long

i am new to this and i am stuck with this error

Undefined property: stdClass::$id in C:\xampp\htdocs\thesisApp\db\login.php on line 6

here is the code

<?php 
$con = new mysqli("localhost", "root", "root", "thesisAppdb");
$data = json_decode(file_get_contents("php://input"));
$id = mysqli_real_escape_string($con, $data->id);
$username = mysqli_real_escape_string($con, $data->username);
$password = mysqli_real_escape_string($con, $data->password);

 $query = ("SELECT id FROM teacherdata WHERE username= '$username' and 
 password= '$password' and id = '$id'");
 $que = mysqli_query($con, $query);
 $count = mysqli_num_rows($que);

 if ($count == 1) {
 echo 'correct';

} else {
echo 'wrong';
}
?>

hope to hear news from anyone.

As you can see from the output of var_dump, that file_get_contents("php://input") contains an empty json object. Which when decoded provides an empty PHP object. So, you need to make sure that there is id key in the json object and on the PHP side you can use the following:

$data = json_decode(file_get_contents("php://input"));
$id = 0;
if(isset($data->id))
{
    $id = mysqli_real_escape_string($con, $data->id);
}