Randomized login background

Hi, I want to change login background (randomized). I've take some ispiration by http://xlinesoft.com/blog/2014/10/31/random-page-background-halloween-edition/

and I've done this code:
Custom CSS

/ login page background /

body.function-login {

height:100%;

background-image:url("http://192.168.2.108/gestione2015v80/output/images/'.$_SESSION["background"].'";);

...
Login page -> Before Display
$bg = array('sfondo01.jpg', 'sfondo02.jpg', 'sfondo03.jpg', 'sfondo04.jpg', 'sfondo05.jpg', 'sfondo06.jpg', 'sfondo07.jpg' );

$i = rand(0, count($bg)-1); // generate random number size of the array

$_SESSION["background"]= "$bg[$i]";
I think the mistake is in the session var. Have U any suggestion to fix my code? <img src='https://asprunner.com/forums/file.php?topicimage=1&fieldname=question&id=22992&image=1&table=forumtopics' class='bbc_emoticon' alt=':rolleyes:' />