Added register_shutdown_function in example

Added `register_shutdown_function()` to prevent unexpected behavior on php versions less than 5.4.0.

Explanation taken from php website:
> Warning: When using objects as session save handlers, it is important to register the shutdown function with PHP to avoid unexpected side-effects from the way PHP internally destroys objects on shutdown and may prevent the write and close from being called. Typically you should register 'session_write_close' using the register_shutdown_function() function.