PHP Fatal Error SessionHandlerInterface

Question

I am trying to implement a custom SessionHandler in PHP, that handles to session through a database using transactions.

The problem though, is that I keep getting a fatal errors, about the function being on null.

" Fatal error: Call to a member function begin_transaction() on null "
" Fatal error: Call to a member function commit() on null "

If I comment out the previous two I get:
" Fatal error: Call to a member function prepare() on null "
" Fatal error: Call to a member function close() on null "

Etc. Etc.

Now obviously, you would think that I forgot to initiate a connection with the database. Well, surprise, I have not.

<?php
    if (!empty($_POST)) {
        $username = trim($_POST['username']);
        $password = trim($_POST['password']);

        require_once '../psl-config.php';
        $db = new mysqli('localhost', USER, PASSWORD, DATABASE, 0, '/var/run/mysqld/mysqld.sock');

        if (!$db->connect_errno) {
            $login = $db->prepare('SELECT hashedPassword FROM gebruikerGegevens WHERE username = ?');
            $login->bind_param('s', $username);
            $login->execute();
            $login->bind_result($hashedPassword);
            $login->fetch();
        }

        if (password_verify($password, $hashedPassword)) {
            require_once 'sessionHandler.php';

            $handler = new MysqlSessionHandler($db);
            session_set_save_handler($handler);
            session_start();
        } else {
            echo "Username or password is incorrect.";
        }
    }
?>

Everything goes well until I initiate the MysqlSessionHandler function. NOTE: I have not inserted the rollback functions, as I still was busy figuring the bug out.

<?php
class MysqlSessionHandler implements SessionHandlerInterface {
    protected $db;

    public function open($save_path, $session_name) {
        return true;
    }

    public function read($session_id) {
        $db->begin_transaction(MYSQLI_TRANS_START_READ_WRITE);
        $db->prepare('SELECT expiry, data FROM sessions WHERE sid = ?');
        $db->bind_param('s', $session_id);
        $db->execute();
        $db->bind_result($expiry, $data);
        $result = $mysqli->fetch();

        if ($result) {
            if ($expiry < time()) {
                return '';
            }
            return $data;
        }

        //Inserts sessionID into database, if no sessionID could be found in the database.
        $db->prepare('INSERT INTO sessions (sid, expiry, data) VALUES (?,?,?)');
        $db->bind_param('sib', $session_id, $create_expiry, $empty_data);
        $create_expiry = session_cache_expire();
        $empty_data = '';
        $db->execute;
        return '';
    }

    public function write($session_id, $session_data) {
        $db->begin_transaction(MYSQLI_TRANS_START_READ_WRITE);
        $db->prepare('INSERT INTO sessions (sid, expiry, data) VALUES (?,?,?)');
        $db->bind_param('sib', $session_id, $create_expiry, $session_data);
        $create_expiry = session_cache_expire();
        $db->execute();
        return true;
    }

    public function close() {
        $db->commit();
        $db->close();
        return true;
    }

    public function  destroy($session_id) {
        $db->prepare('DELETE FROM sessions WHERE sid = ?');
        $db->bind_param('s', $session_id);
        $db->execute();
        return true;
    }

    public function gc($maxlifetime) {
        return true;
    }
}
?>

Answer 1

The $db variable is an instance variable, which means it must be referenced using $this .

Example

$this->db->begin_transaction(...);

But before being able to reference $db in this way, you have to set it using the constructor method.

class MysqlSessionHandler implements SessionHandlerInterface {
    protected $db;

    public function __construct($db) {
        $this->db = $db;
    }

    public function read(...) {
        ...
        $this->db->begin_transaction(...);
        $this->db->prepare(...);
        ...
    }
}