What is the difference between session_unset() and session_destroy() in PHP?

From the php.net documentation:

session_destroy — Destroys all data registered to a session

session_unset — Free all session variables

My three part question is:

The two functions seem very similar.
What is really the difference between the two?

Both seem to delete all variables registered to a session. Does any of them actually destroy the session itself? If not, how do you accomplish this (destroy the session itself).

Is it correct that neither of the two functions deletes the session cookie at the client?

session_unset just clears the $_SESSION variable. It’s equivalent to doing:

$_SESSION = array();

So this does only affect the local $_SESSION variable instance but not the session data in the session storage.

In contrast to that, session_destroy destroys the session data that is stored in the session storage (e.g. the session file in the file system).

Everything else remains unchanged.

session_destroy(); is deleting the whole session.

session_unset(); deletes only the variables from session - session still exists. Only data is truncated.

session_unset();

Just clear all data of all session variable.

session_destroy();

Remove all session.

session_start();
session_destroy();     
$a = "1234";
$_SESSION[a] = $a;

$_SESSION[a] is NULL.

session_start();
session_unset();     
$a = "1234";
$_SESSION[a] = $a;

$_SESSION[a] is 1234.

So, I will use:

session_start();
session_destroy();   
session_start();  
$a = "1234";
$_SESSION[a] = $a;