I rotate a banner on my site by selecting it randomly from an array of banners.

Sample code as demonstration:

<?php
$banners = array(
'<iframe>...</iframe>',
'<a href="#"><img src="#.jpg" alt="" /></a>',
//and so on
);
echo $banners(rand(0, count($banners)));
?>

The array of banners has become quite big. I am concerned with the amount of memory that this array adds to the execution of my page. But I can't figure out a better way of showing a random banner without loading all the banners into memory...