difference between unset and destroy session in php code example

Example 1: php delete session

session_destroy(); // To delete whole session
// OR
unset($_SESSION['myVar']); // To delete a session var

Example 2: destroy session php

<?php 
 session_start(); // start session
 session_destroy();  // Delete whole session
// OR
unset($_SESSION['username']); // delete any specific session only
?>