$username = trim($_POST['username']);
$password = trim($_POST['password']);

if (isset($username) AND isset($password)) {
    $db = new DBOperations();

    if($db->userLogin($username, $password)) {
        $user = $db->getUsernameByUsername($username);
        $response['error'] = false;
        $response['message'] = "succes";
    }else {
        $response['error'] = true;
        $response['message'] = "Invalid username or password";
    }
}else {
    $response['error'] = true;
    $response['message'] = "Required fields are missing";
}

public function userLogin($username, $pass) {
            $stmt = $this->conn->prepare("SELECT id FROM students WHERE s_id = ?");
            $stmt->bind_param("s", $username);
            $stmt->execute();
            $stmt_result = $stmt->get_result();
            if($stmt_result = $stmt->num_rows > 0) {
                while($row_data = $stmt_result->fetch_assoc(MYSQLI_ASSOC)) {
                    # Action to do
                    echo "success";
                }

            } else {
              # No data actions
              echo 'No data here :(<br/>';
            }
        }