PHP Pan-resolution two-level domain name session of the different steps of the problem seeking angle

PHP Pan-resolution two-level domain name session in different steps of the problem seeking angle
Login from the main domain account, enter the two-level domain name session is gone. But logins from the two-level domain name can be synced to the primary domain. Different two-level domain names can be logged into different accounts. How to solve the problem of kneeling. The ability to use the database without the database is as far as possible.

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Session_set_cookie_params specifying the scope of the PHPSESSID

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