Let open, close, read, write, destroy and gc be PHP functions to save
the superglobal array $_SESSION in a database.

I would like to use the following code:

[1] session_set_save_handler(open,close,read,write,gc) ;
[2] session_start();
[3] if(!isset($_SESSION))
[4] {
/* error handling */
exit;
}
/* session okay */

But what will happen, if (e.g.) the database is down?
Means: What kind of error handling can or must be implemented within the
functions open, close, read, write, destroy and gc to get a stable
script? Of course, line [3] is not enough ...