I just can’t destroy a session in php

When I logout by destroying the session and start another one it always shows me the first session info I started $_SESSION[‘username’];

What I mean here is that : I started a session the first time I logged with this username –> AAAAAA And destroyed the session using the file logout.php which contains this code :

session_start();
session_destroy();
header("location: login.php");

and login with another username —> BBBBBB and it always shows me the first username I logged in with—> AAAAAA

Where is the problem here

Here is the code (login.php)

<?php
        session_start();
        require_once "config/db.php";
        if(isset($_POST['login'])){     
        $username       = trim(mysql_real_escape_string($_POST['username']));
        $password       = trim(mysql_real_escape_string(md5($_POST['password']))); 
        $query          = mysql_query("SELECT * FROM `users` WHERE username='$username' AND password='$password' ")  or die(mysql_error());
         $rows           = mysql_num_rows($query);

        if($rows == 1){

            while($info = mysql_fetch_object($query)){            
                $dbusername = $info->username;
                $dbpassword = $info->password;
            }

            if($dbusername == $username && $dbpassword == $password){                   
                header("Location: index.php");         
                $_SESSION['username'] = $username;  
            }       

        }else{

        }
    }
?>

the index.php :

  <?php session_start();?>
  <h3><?php echo $_SESSION['username']; ?></h3>

Answer

From docs:

In order to kill the session altogether, like to log the user out, the session id must also be unset. If a cookie is used to propagate the session id (default behavior), then the session cookie must be deleted. setcookie() may be used for that.

Example:

session_unset();
session_destroy();
session_write_close();
setcookie(session_name(),'',0,'/');
session_regenerate_id(true);