`$_OLD_SESSION` isn't set if `$_SESSION` isn't set before `session_start()` (with the code that this commit changes) and if `$_OLD_SESSION` isn't set then you're serializing a non-existent variable, which causes a `Notice: Undefined variable:  _OLD_SESSION` E_NOTICE on PHP 4.4, 5.0, 5.1 and 5.2.