when and where do we use session_id()

Question

I can't understand the code below and I don't know when we use session_id() before session_start() .

<?php
if($_GET){
    //defining the session_id() before session_start() is the secret
    session_id($_GET['session_id']);
    session_start();
    echo "Data: " . $_SESSION['theVar'];
    //use your data before below commands
    session_destroy();
    session_commit();
}else{
    //common session statement goes here
    session_start();
    $session_id=session_id();
    $_SESSION['theVar'] = "theData";
    echo "your.php?session_id=" . $session_id;
}
?>

i want you to explain it ! not just copying the description of php.net !
on the other hand , where is session_id() used ?! what's its usage ?! thank you in advance !

Answer 1

finally i understood ! i give you two examples :

<?php 
session_start();
session_id();
?>

result |stbug36ge9efg20cpdjnq83m50 ( session id )

and whenever the browser or tab is closed , the session will be omitted and the next time you enter the site , you can manage two action to occur : 1. start a new session with previous session_id 2. or start a new session witt a new id as the usual , the action num.2 will happen but if you want num.1 to happen you have to embed session_id before session_start . look at the code below :

<?php
session_id("stbug36ge9efg20cpdjnq83m50");
session_start();

?>

and here we're starting a new session with previous session id .

and

the usage of Session_id()

you can easily write a online visitors counter -- each time a session starts(use is online ) , its id will store in database . so we can find out how many users are online.