How to use sessions with scripts called from php_include

[Aurous]

Here's the simple script (banner.php) I made to rotate banners.

<?php
srand((float) microtime() * 10);
 
$input = array(
"<img src=\"$stylevar[imgdir_misc]/banner1.jpg\" width=\"480\" height=\"60\" />", 
"<img src=\"$stylevar[imgdir_misc]/banner2.jpg\" width=\"480\" height=\"60\" />", 
"<img src=\"$stylevar[imgdir_misc]/banner3.jpg\" width=\"480\" height=\"60\" />" 
);
 
$rand_keys = array_rand($input, 3);
 
echo $input[$rand_keys[0]] . "\n";
 
?>

and this what I have in php_include:

ob_start(); 
include_once("banner.php");
$randomlogo = ob_get_contents(); 
ob_end_clean();

Now the question I have is what should I change in the script so it only run once every user session? Please help.

Thanks

Aurous