How does PHP use session to pass values on two pages in the same browser?

Add the following at the beginning of the first pageCode:
<? PHP
Session_start ();
$ Current_id = session_id ();
Var_dump ($ current_id); // print the session information of $ current_id
$ _ Session ['username'] = 'good ';// Set the username value of the session variable.
?>
Add the following to other files of the same project to access the value of the session variable set above.
<? PHP
Session_start ();

Var_dump ($ _ session );// Print the session information

$ Name = $ _ session ["loginusername"];

?>