Ask a session very simple about the question, little brother where I might be trapped

Ask a simple question of the session, where I might have been trapped.
PHP uses the session value to determine whether the user is logged in, I often see this example:
is to have a form, enter a user name and password, and then verify on a PHP page, if the validation is successful (that is, if a user name and password are found to match from the database), generate the following sessoin
$_session[' username '] = ' csdn ';
$_session[' pwd '] = ' pwd ';


Then on the other page to determine whether $_session[' username ' is empty, if not empty, the description is logged in ...

Then my question comes: Does the $_session exist on the server? That many users logged in, $_session[' username ' is not to be reset many times, how the server correctly to the client to make a distinction???

I am dizzy, I feel very simple thing, the result I Leng want not to understand ... It must be my understanding of the wrong, please help me to point out.
Thank you.
------to solve the idea----------------------
$_session is generated by the Session_Start function
The content of $_session is session_start based on the value of the incoming cookie.
So there's no doubt in your case.

That is to say user A's $_session and User B's $_session Although all is $_session, but the content is not the same
------to solve the idea----------------------
Session will have a session_id from the client to identify different users, this session_id is unique, so landlord do not worry about confusion.
------to solve the idea----------------------
Each call is unique ... That is the session_id mentioned above, so every time the browser and the server to establish the call when the ID is unique, the server will determine who I am talking to.

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