It should be fixed to check all required methods are implemented or not.

It should not call simply call old save handler functions essentially when it's a module provided functions. It does not make sense at all to call module save handler.

Current implementation can use some what like class inheritance, e.g. call previously set save handler, I don't know users are using this behavior. For example,

session_set_save_handler($myBaseSessionHandler);
session_set_save_handler($myExtenedSessionHandler);
session_start()

Rather than this. User should use class inheritance

class myExtendedSessionHandler extends myBaseSessionHandler {}

Not like inheritance, when nest level is more than 1, ancestor class method is not called.

session_set_save_handler($BaseSessionHandler); // Useless
session_set_save_handler($myBaseSessionHandler);
session_set_save_handler($myExtenedSessionHandler);
session_start()

It's better to remove this behavior. IMO.

I wander why current implementation does not check implemented interfaces.

I made session module abuse harder. Closed.