Questions about cookies and session logins

Cookies and Session login issues
There are a few questions to help you answer

1. How to determine if the user is logged in after successful login
I wrote the session username and password to the database in the correct comparison to count the login
Some say it's just a matter of determining if a session exists, even if login does not exist jump to login page
is the second method safe? Can the session and the cookie be forged directly on the login?

2. I only used the session login, now want to increase the automatic storage of cookies login function, the session still need to use it?
What's the whole idea?

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To determine if you are logged in, just determine if there is a record in the session.
If you are worried about a fake session, then you have the user name and password in the session will not worry about forgery and leaks?

Need or need, after all, cookies are stored in the client, but also in the network transmission, the probability of being stolen is very large

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Explore

A friend gave me a way
Just add the following sentence after the set session.
Setcookie (Session_name (), session_id (), time () + 2592000, "/");

But I don't quite understand the meaning of this remark.

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Automatic login requires the use of cookies, or how the server knows who is who?
The cookie requires a user name with an expiration period, and an identity string that can be processed by the user's information.
When PHP receives a cookie, it thinks that the user is logged in
If verification is required, the user's identity string is computed and the incoming identity string is compared

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