public function login($nick, $email, $password)
{

    $query = "
            SELECT
              *
            FROM
               " . $this->table_name . "
            WHERE
                email = ?
            OR
                nick = ?
         ";

    // prepare query statement
    $stmt = $this->conn->prepare($query);

    //bind values from user
    $stmt->bindParam(1, $email);
    $stmt->bindParam(2, $nick);

    $stmt->execute();

    if($stmt === true) {
        $row = $stmt->fetch(PDO::FETCH_ASSOC);
        $num = $stmt->rowCount();

        if ($num === 1) {
            if (password_verify($password, $row['password'])) {
                $token = array();
                $token['id'] = $row['id'];
                $encoded_token = JWT::encode($token, 'secret_server_key');

                $query = "
                    UPDATE
                        " . $this->table_name . "
                    SET
                        token = " . $encoded_token . "
                    WHERE
                        id = " . $row['id'] . "
                ";

                $stmt_token = $this->conn->prepare($query);
                $stmt_token->execute();

                if ($stmt_token === true)
                {
                    http_response_code(200);
                    return true;
                }
            }
            else
            {
                http_response_code(402);
                return false;
            }
        }
        else
        {
            http_response_code(401);
            return false;
        }
    }
    else
    {
        http_response_code(400);
    }
}

<?php
    if(isset($_POST['submit']))
    {
        if(
            (
                (isset($_POST['nick']) && !empty($_POST['nick']))
                    ||
                (isset($_POST['email']) && !empty($_POST['email']))
            ) &&
            (isset($_POST['password']) && !empty($_POST['password']))

        )
        {
            // instantiate database
            require_once '../config/database.php';
            $database = new Database();
            $db = $database->getConnection();

            require_once '../classes/user.php';
            $user = new User($db);

            if(!isset($_POST['nick']))
            {
                $user_nick = null;
                $user->login($user_nick, $_POST['email'], $_POST['password']);
            }
            else if(!isset($_POST['email']))
            {
                $user_email = null;
                $user->login($_POST['nick'], $user_email, $_POST['password']);
            }
        }
        else
        {
            http_response_code(400);
        }
    }
?>