Using SessionHandlerInterface with MySQL database ...please advise

Using SessionHandlerInterface with MySQL database ...please advise

I have made the following code by following
http://www.php.net/manual/en/class.sessionhandlerinterface.php
AND
http://www.wikihow.com/Create-a-Secure-Session-Managment-System-in-PHP-and-MySQL
Here I am using MySQL database to store and retrieve session variables.
This code works fine. However it would be great if you are kind enough to
point out the mistakes and share your input about this code.
class MySessionHandler implements SessionHandlerInterface
{
public function open($savePath, $sessionName)
{
$host = 'localhost';
$user = '******';
$pass = '******';
$name = '*******';
$mysqli = new mysqli($host, $user, $pass, $name);
$this->db = $mysqli;
return true;
}
public function close()
{
return true;
}
public function read($id)
{
if(!isset($this->read_stmt)) {
$this->read_stmt = $this->db->prepare("SELECT data FROM sessions
WHERE id = ? LIMIT 1");
}
$this->read_stmt->bind_param('s', $id);
$this->read_stmt->execute();
$this->read_stmt->store_result();
$this->read_stmt->bind_result($data);
$this->read_stmt->fetch();
$key = $this->getkey($id);
$data = $this->decrypt($data, $key);
return $data;
}
public function write($id, $data)
{
// Get unique key
$key = $this->getkey($id);
// Encrypt the data
$data = $this->encrypt($data, $key);
$time = time();
if(!isset($this->w_stmt)) {
$this->w_stmt = $this->db->prepare("REPLACE INTO sessions (id,
set_time, data, session_key) VALUES (?, ?, ?, ?)");
}
$this->w_stmt->bind_param('siss', $id, $time, $data, $key);
$this->w_stmt->execute();
return true;
}
public function destroy($id)
{
if(!isset($this->delete_stmt)) {
$this->delete_stmt = $this->db->prepare("DELETE FROM sessions WHERE
id = ?");
}
$this->delete_stmt->bind_param('s', $id);
$this->delete_stmt->execute();
return true;
}
public function gc($maxlifetime)
{
if(!isset($this->gc_stmt)) {
$this->gc_stmt = $this->db->prepare("DELETE FROM sessions WHERE
set_time < ?");
}
$old = time() - $max;
$this->gc_stmt->bind_param('s', $old);
$this->gc_stmt->execute();
return true;
}
private function getkey($id) {
if(!isset($this->key_stmt)) {
$this->key_stmt = $this->db->prepare("SELECT session_key FROM
sessions WHERE id = ? LIMIT 1");
}
$this->key_stmt->bind_param('s', $id);
$this->key_stmt->execute();
$this->key_stmt->store_result();
if($this->key_stmt->num_rows == 1) {
$this->key_stmt->bind_result($key);
$this->key_stmt->fetch();
return $key;
} else {
$random_key = hash('sha512', uniqid(mt_rand(1, mt_getrandmax()),
true));
return $random_key;
}
}
private function encrypt($data, $key) {
$salt =
'cH!swe!retReGu7W6bEDRup7usuDUh9THeD2CHeGE*ewr4n39=E@rAsp7c-Ph@pH';
$key = substr(hash('sha256', $salt.$key.$salt), 0, 32);
$iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_ECB);
$iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);
$encrypted = base64_encode(mcrypt_encrypt(MCRYPT_RIJNDAEL_256, $key,
$data, MCRYPT_MODE_ECB, $iv));
return $encrypted;
}
private function decrypt($data, $key) {
$salt =
'cH!swe!retReGu7W6bEDRup7usuDUh9THeD2CHeGE*ewr4n39=E@rAsp7c-Ph@pH';
$key = substr(hash('sha256', $salt.$key.$salt), 0, 32);
$iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_ECB);
$iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);
$decrypted = mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $key,
base64_decode($data), MCRYPT_MODE_ECB, $iv);
return $decrypted;
}
}
$handler = new MySessionHandler();
session_set_save_handler($handler, true);
session_start();
Thanks for your time
Cheers