As of PHP 5.0.5 the write and close handlers are called after object destruction and therefore cannot use objects or throw exceptions. The object destructors can however use sessions.

It is possible to call session_write_close() from the destructor to solve this chicken and egg problem.

If using mysqli in procedural style you may expect the mysqli handle to be valid in the write callback. It is not the case, the handle in procedural style is a object that will be destroy unlink the plain mysql interface handle. Just register a shutdown function after set save handler like this:

session_set_save_handler('_session_open', '_session_close', '_session_read', '_session_write', '_session_destroy', '_session_gc');
register_shutdown_function('session_write_close');