MySQL connection fails because of invalid link resource - php

Question

My Code:
class cdbsHandler
{
[...]
private $link;
function __construct($mysqld)
{
[...]
$this->link = mysql_connect(
$mysqld['host'], $mysqld['user'], $mysqld['password']
);
mysql_select_db($mysqld['database'], $this->link);
}
function write($sessionId, $data)
{
$sessionId = mysql_real_escape_string($sessionId, $this->link);
}
[...]
}
I use the class like this:
static function startSession($name, $mysqld)
{
[...]
$sessionHandler = new cdbsHandler($mysqld);
session_set_save_handler(
array ($sessionHandler,"open"),
array ($sessionHandler,"close"),array ($sessionHandler,"read"),
array ($sessionHandler,"write"),array ($sessionHandler,"destroy"),
array ($sessionHandler,"gc")
);
session_start();
}
Where $mysqld are the database credentials.
The static function is called every time that a user loads a page.
The error I get is:
mysql_real_escape_string(): 2 is not a valid MySQL-Link resource
(calling mysql_real_escape_string in the write function)
The credentials are correct. They are loaded from a config file and are successfully used to establish a connection a few lines after the session was started.
What seems very important to me:
I am actually able to login and data is written to the database (/session) but as soon as the login page redirects me to the home page the error mentioned above appears.
(The login page verifies a form token that is saved in the session data. But after sending that request the script isn't able to access any session data anymore.)
More Information:
The code is from a memcached session handler with database "backup".

Answer 1

Most likely this is what's happening (best guess based on your information):
Per the manual the session write handler is not called until after the script has ended. In many versions of PHP, all class destructors are run before calling the session write handler. Consequently, if you are closing the connection in __destruct(), then __destruct() is called before write() is - leading to an invalid connection when you try to save the session.